As your character goes on adventures and overcomes obstacles, they gain experience in the form of character points. When your CPT reaches a certain amount, your character becomes stronger and gains new abilities. This is the process of leveling up.
The rate at which you gain character points depends on the progression mode being used by the GM. They will notify you, either during or between sessions, when your character levels up.
In many tabletop games, choosing your class is one of the first decisions you make when creating a new character. In mRPG-based games, however, new characters start out Classless. You may choose a class the first time you level up, or continue to level in Classless until you decide which class you want to take.
Your overall level is the total number of times you've leveled up, no matter which classes you applied those levels to. Your class levels are the number of levels you've put towards each class you have. This can differ from your overall level if you applied some levels to Classless, or if you're multiclassing.
Pay close attention to the difference between these when looking at feats. If a class feat requires Level 7, for instance, it requires 7 levels in that class, not overall.