XP (short for eXperience Points) can be earned through:
- Daily Challenges (prior to Season X)
- Weekly Challenges (prior to Season X)
- Missions (since Season X)
- Medals (since C2-S1)
An accurate XP table can be found here.
XP has been in the game since the beginning. By playing the game, you can level up in order to get rewards. In C2-S1, XP became really important in progressing the Battle Pass.