"Practice makes perfect and there is no better practice than field duty." - Joe
Campaigns are chains of missions. Your goal is to complete unique missions objectives and earn as many stars as possible. The more stars you collect, the bigger the reward. Once you begin the Campaign a timer starts. This timer is counting down the time you have left for completing the campaign. Once you are done, another timer counts down the time until another campaign becomes available.
Depending on your overall score you can earn rewards like free chests with awesome loot.