"The file is a decompression bomb"

A decompression bomb is a file that unpacks to an enormous amount of data - thus "flooding" the unpacking engine. It's quite hard to detect such files reliably, so it's possible that it gives some false alarms ocassionally.

Typically such a bomb is a multi-level packing thing -- data's compressed with one packer (e.g. into a zip), then the resulting archive file is in turn packed (usually with a different packer), and so on several times.

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.