Oh, it's MUCH easier than Fusions. Here's a small tutorial:
If you have a Level...7 Synchro, for example, you can only (this is all normally, btw) summon it by having a Tuner on the field (kind of monster), and at least 1 non-tuner (the regular monsters) that's levels equal 7.
So, I have a Level 3 Tuner, and a Level 4 Non-Tuner, I can summon a Level 7 Synchro with just those two monsters.