Well the most basic thing about synergy combos is this: you or someone else has a skill that other people can use along with it to make it stronger. For instance, sorcs have a summon atronach skill that once used, another nearby player can hit 'x' (by default) and make the damage done more powerful. There are tons of these skills in the game, and when you can use one, a pop-up will appear on your screen telling you to hit 'x'.

Hope that helps.

this is like a main thing for this game and theres hardly any info on it and of the devs know?