Perk is a bonus specialization of your character, most often in the form of a bonus to some of your statistics (eg. BonusHthDamage or EarlierSequence).
You can choose a new perk every three levels (or every four levels if you chose the Skilled trait when creating your character).
Many perks have a LevelRequirement or an AttributeRequirement (or both).
Some perks have multiple ranks (can be obtained multiple times).
⚠️ FutureContent as of version 2021.04.19::001. Perks exist behind the scenes but cannot yet be chosen even if you obtain level 3.
List of currently implemented perks:
There are more perks in the original Fallout 2 game - they will most likely come with later game updates.