Each slider on an equalizer represents a range of frequencies, from the lowest on the left to the highest on the right, whose volume can be raised or lowered independently of the others, so the listener can alter the music's overall sound to taste. Good rules of thumb when first adjusting an equalizer are to start with every bar in the zero, or middle position, and to generally decrease levels rather than raising them, making one adjustment at a time.
Another general rule is that no frequency band should typically be moved very far from it's neighbor on either side, as this can sound very unpleasant to most people and possibly distort the music. The sliders should also almost never be raised all the way up, either individually or all together, as this almost certainly "clips" the volume past the available range and can be physically damaging to components of the speakers or headphones, in addition to distorting terribly.
With a little practice, anyone who can hear can learn to target an annoying element or weak link in a song and adjust the equalizer to compensate. However, overcoming the limitations of poor or damaged speakers is impossible. For example, the lowest slider on most equalizers is around 20-60 Hertz, a range known as sub-bass, which small speakers invariably cannot reproduce. No amount of amplification in this region makes the sub-bass more prominent if the audio is playing through speakers whose frequency response bottoms out at 55 Hz; it is likely more beneficial to attenuate this range on small speakers to add clarity to the mix.