This is a super over-simplification but here goes:
Traveller takes the boom left/right, mainsheet takes it up/down.
If you've sailed a smaller boat without a traveller, you probably used the mainsheet to move the boom left and right and didn't worry too much about up or down. You can use a combination of both to achieve this all important twist thing that folks mentioned above and is worth reading about and practicing alot.
In the meantime, trimming with the traveller to move the boom left and right will probably be the most familiar maneuver, in effect.
I am newbie too, so if I am dead wrong any corrections from saltier ones will help me too!