Experience is a reward that you receive for completing various actions in the game, such as successfully investigating a crime scene and completing quests. Your experience is shown as gold stars. You can see the total number of stars you've earned in your player profile.
Once you accumulate a certain amount of experience, you level up. This grants new opportunities, like access to new investigations, emergencies, and other in-game activities.
Additionally, every time you level up, your maximum supply of energy increases.