A block brake system is used to prevent roller coaster trains from colliding with one another. Almost all modern roller coasters which operate multiple trains have a block brake system in place. A mid-course brake run is a block brake found in the middle of a ride's course.
The track of a roller coaster is divided up into sections, called "blocks" or "block sections", by brake runs which are capable of stopping the train. The control system ensures that only one train is allowed into each block section at a time. If a train tries to enter a block section occupied by another train, the ride control system will instead stop the train on the brake run. Holding a train mid-ride may give the impression that the coaster is unsafe or "broken down" when it is in fact the control system working as intended.
Roller coaster block systems work on the same principles as those found in railways - the track is divided up into sections which are only allowed to hold one train.