Missions are a feature in the game that allows a player who is logged in to complete two different missions to earn XP for the Survivr Pass.
You can change a mission by clicking the refresh arrow in the upper-right corner of each mission. This has a cooldown of 1 hour before you can change your missions again for free.
There is also the option to refresh a mission before the cooldown is over. You can do this either spending some GP or, if you're using the mobile app, by watching an ad.