What is a "decompression bomb"?

I ran a full scan. The Results showed that some files could not be scanned and indicated that a few were “decompression bombs.”

I looked in Help and searched the Forum, but I couldn’t find an easy explanation. So, . . . what is a “decompression bomb”? Is it a virus? Malware? Something else bad that needs to be dealt with?

Thanks.

Quote from Igor…

- decompression bomb is just something that unpacks to an unusually big amount of data even though it's rather small (i.e. has a high compression ratio, for example). It's nothing to worry about, you are just informed that avast! will not try to unpack the archive (you may not even know that it's an archive, but it seems like it is) because it may take VERY long to process.