A flock is a coagulant which binds suspended particles in the water and drops them to the bottom of the pool. When this process is carried out it leaves the water clear to inspect the bottom.
The down side of this is it can be difficult to remove the flock if you don’t have a way to bypass your filter. It can be an effective way to recover a pool if the process is carried out correctly.
As a general rule, a flock is less likely to work if an algaecide has already been added to the Pool