In the Career mode, you can proceed by gaining fans. There are 100 steps in the Career. As you win fans, you progress in the Career and get a reward at each step.
If you lose fans, your Career progress will also decrease. Rewards in each step can only be collected once, meaning that when you regain fans, you will not be able to collect the rewards you previously collected.
You can use fan boosts to progress faster in the Career.