Some languages, such as Java, use garbage collectors that periodically go through the heap and recycle chunks of memory that are not being used. That does not happen in a C++ program. If you lose all pointers to a chunk of memory without deallocating that memory then you have a memory leak. Your program will continue to own that memory, but has no way of ever using it again. A very small memory leak is not a problem. But if a program allocates a lot of memory and never deallocates it, the program's memory footprint will grow and grow.
When your program stops, all of the memory that it owns is automatically recovered by the operating system. So a memory leak is only relevant while a program is running; it does not affect the system after the program stops.