Backmarkers in F1 refer to the cars falling behind in the race and they are often lapped by the fastest cars. There have been incidents where a backmarker collided with leading drivers on track.
Blue flags are raised to inform the backmarker that he should pass a leading car. Usually, there are two, or three teams that become backmarkers in each F1 season. There is a disparity between the cars used, and drivers’ and teams’ performance that leads to the race having backmarkers.
Each F1 team makes its vehicle from scratch, so it’s common there will be lapped cars and leading cars.