After every mission you'll get different kinds of experience points (XP) and ability points to spend in specific ways that boost either your support characters or the hero characters you play as. You'll also unlock character cards that let you assign support characters to your team or hero cards that let you play as different characters with various combat or support specializations. Likewise, you'll find item schematics that let you build new weapons or traps as well as improved versions of those items when out in the field. There's even a collector's book that you can fill out with duplicate cards to unlock even more rewards.
Again, it can be a lot to Cheap Fortnite Items sift through in the beginning— basically, everything can be leveled up with some kind of specific XP currency, and there’s a lot of different stats to ogle— but the game doesn't push you too hard in the beginning, so don't sweat these decisions too much at first.