Welcome to the forums Mindy, this thread is not related to your question so you should really have started a New Topic for questions unrelated to the topic.
But to briefly answer your question, it is a file that appears small but when uncompressed it becomes very large, this is a common tactic by some malware to swamp systems thereby allowing access or crashing it. It doesn't mean this is definitely harmful just airing on the side of caution of not decompressing it in order to scan it.
Also a forum search for decompression bomb should return some information as this topic has been discussed before.
