A battle pass is a system for monetization that is often included, but not limited to free-to-play games. This system often includes a tier system that requires players to play the game for an amount of time, get XP, or do specific challenges in order to level up.
Battle passes are often available for a limited amount of time, encouraging players to come back often in order to not miss out.
Some games offer both a premium and free path through a battle pass. The free tiers usually give much smaller or more infrequent rewards to the premium path.