A memory leak is when a computer program doesn't properly free up the memory it was using to run. Typically this results in the program crashing after it uses up all available computer memory and cannot run anymore.
In Maplestory, this is usually indicated by missing assets, overlapping assets, skills not hitting properly, and weird BGM issues, typically followed by the game crashing.
This is usually fixed by doing a restart or a full shutdown of your computer. In extreme cases, uninstalling and reinstalling might help as well.