What do equalizer controls provide in db? What do equalizers (like New Klark Teknik | High-Performance Dual-Channel 30-Band 1/3 Octave Analogue Graphic Equalizer, DN370 with Low/Hi Pass and Notch Filter) mean when they say +6 and -6 on the y axis? Is that 6db?
Yes. That means whatever frequency is being affected by the equalizer is being changed by the number of decibels indicated. Most of the time, a change of 3 to 6 decibels at a frequency is audible to most listeners. During mastering of recorded music, the mastering engineer may make important changes of .5 to 1.5 decibels or smaller.
Don't let any of the technical aspects distract you from basically making it sound good to your ears. If you are trying to make music sound good to others, you have to start with your own ears and learn how they are different from others' ears, and the best way is to get in there and start making changes and seeing how those changes sound.