The acquisition and advancement of your heroes determines your Squad Level.
Your Squad Level increases as your heroes rise in level. For each hero, the squad level increases by the following formula:
Marvel Lore + Hero Level + 1 (if maxxed)
Ex: If you buy Gambit, he is Lore +1 and he starts at level 1, so your overall squad level would increase by two.
Raising your Squad Level will allow you to collect squad Achievements, which will earn you rewards.
Each character's Marvel Lore number can be found in the Character Selection window (or by looking it up on this wiki). The range is 0–3.