During each race you will be given the opportunity to complete a “bonus objective” for extra rep. They are usually easy, and involve travelling at fast speeds, completing sprints/laps fast, or drifting. Each car for each event has a different objective, and only 1 objective per race. You can view the objective before the race, at the pre-race menu. It is located in the white stripe of text underneath the circuit picture. You can also pause during a race to view to goal. Complete the goal in 25 different races, and you will unlock this achievement.
Note: You can complete 2 bonus objectives for rally events that have 2 stages.