I watched your videos, but I didn't understand completely what a graphic eq does or do I need it at all? There are four types of eqs. For a quality song, I don't need to use them all.. ?

You don't need to use any EQ, unless your ear tells you that the tone of some audio needs to be adjusted. For example, if a drum is too midrangey, you might want to cut (reduce) the mids a little; if a bass is too thin you might want to boost the low frequencies; if a guitar is too bright, you might want to gently reduce the level of the upper midrange and treble, etc.. A parametric EQ is the most flexible, because you can select the exact frequencies you want to EQ (with a graphic EQ you have fixed frequencies you have to work with). You certainly don't need to use more than one EQ on a track -- typically a three or four-band parametric should be all you need. However, to use an EQ well you have to train your ears to recognize the tonal character in different frequency ranges and learn what the audible effect will be of applying EQ at different frequency bands, and that takes time and practice..

Please explain, what four-band means? I mean a band, in my dictionary band is a music group..

When the term "band" is used in relation to EQ, it means a range of frequencies. An EQ consists of several controls, each of which boosts or cut a specific range of frequencies [within the audible frequency range of 20Hz-20kHz]. Each individual control, acting within its own frequency range, is a band. So an EQ with 4 separate controls for boosting or cutting 4 different frequency ranges would be a 4-band EQ. You can watch the videos on EQ in this course: https://www.macprovideo.com/video/audiopedia-mixing-mastering/6-6-eq-equalization-parametric-eq?afid=E470KLQ7r9 Or for a more detailed explanation, you could watch videos 2,3,&4 in this course [it shows the EQ in Logic, but its mostly the same EQ as the one in GarageBand]: https://www.macprovideo.com/video/apple-logic-pro-201-the-eqs-and-filters/2-2-eq-basics-channel-eq?afid=E470KLQ7r9 Or for a more detailed explanation of EQ you could watch videos 8,9,10,11,12,&13 in this course [it shows different EQs, but the basic concepts apply to all EQs]: https://www.macprovideo.com/video/audio-processing-basics/8-8-eq-basics?afid=E470KLQ7r9 But the best way to learn about EQ is to practice with it yourself. If you go to the Smart Controls panel in GarageBand and click on the EQ button, you'll see an 8-band EQ -- you can experiment with boosting and cutting at different frequencies to hear what it can do. If you have a recording that you think has too much tone or not enough tone at a certain frequency, you can change the tone with the EQ. Ultimately, you'll have to experiment with EQ a lot to train your ears to get familiar with it, and to learn when and how to use it effectively.

I opened the EQ in the smart controls, but I can't see 8-band EQ there, just a picture, here is a video https://youtu.be/8eTEMDHfpsg Or how that 8-band have to look like? And in your educational video I see knobs? or how do you call them, but not in my EQ, video is enclosed. Where are my knobs... ? https://youtu.be/uGS6cSLC9vk

The GarageBand EQ display does not have knobs, it's just a graph -- you drag on the dots to boost or cut, and to change the frequency of each band. Or, once you have selected a band [clicked on a dot] three numerical parameters will show the settings for that band at the bottom -- you can drag them or type in the value[s] you want. The "picture" you refer to in your 1st video IS the GB EQ. Did you watch the videos on EQ I suggested? A lot of of them show the Logic EQ in use -- it's the same as the GarageBand EQ, except that the GB EQ only shows the numerical displays for one band at a time [the currently selected band], while the Logic EQ shows them all. The EQ in your 2nd video is not the GB/Logic EQ, it's a third-party EQ plug-in. Did you watch the rest of that video? At 3:11 it shows how to adjust the settings for the GB/Logic EQ.. https://www.macprovideo.com/video/apple-logic-pro-201-the-eqs-and-filters/2-2-eq-basics-channel-eq?afid=E470KLQ7r9

If parametric eq is most flexible and you also wrote that "typically a three or four-band parametric should be all you need" and I understand of course what more flexible than more choices, then what sense of other three eqs have? I understand, rest of three has less choices, right?

What other three EQs..? Did you mean why would you need more than three or four bands? Well, of course, there might be times when you might find additional bands useful, but most of the time three or four bands would probably be enough. Keep in mind that in the GB EQ some of the bands are different -- the middle four bands [3-6] are parametric [boost or cut everything around the chosen frequency], while bands 2 & 7 are shelving [boost or cut everything above or below the chosen frequency], and the outer two bands are filters [remove everything above or below the chosen frequency]. So that EQ is really a 4-band parametric EQ combined with a 2-band shelving EQ and 2 bands of filters. That provides a lot of flexibility, but of course you don't have to use all those bands at the same time [and normally you wouldn't].

Those other three EQs, you are telling on the video https://youtu.be/0TE_I_064yU

Those are just alternative ways of configuring an EQ, in addition to a full -featured parametric EQ, which is the most most common type of EQ for mixing. The next two videos in that course explain all those different types of filters/EQs, how they're configured, and where/how they'd typically be found/used..