Features are elements of your character that do not fit nicely into numbers. They can add new capabilities, or adjust or augment abilities that you already have. Over the course of your character's career, you will receive many features. Some will be granted directly by your ancestry, back-story or class; others will be chosen by you, either from the list provided by your class, or from the list of General features.
As a rule of thumb, you may not select any feature more than once. Further, it is the name of the feature that distinguishes it, not its origin; if you have two classes (through Hybrid Training, or because you selected the Hybrid class), and both classes offer a feature with the same name, you can only take that feature once.