When program allocates some space is memory, it is used for awhile and when is stopes being used it is deallocated (freed) so that other programs can use this.

Memory leaks happem due to bug in program, which forgets to deallocate some of that space when it stops being used.

This leads to slowdown of the program after some time of it use, since there is more and more in time, allocated space not used for anything.