The chest is a disk location (normally under the avast folder) where the file is moved so it can't do any harm -- similar to Ad-Aware's "quarantine", if you're familiar with that.
You'll also find in the chest a small number of legit (i.e., clean) copies of Win system files -- those are there as backup copies in case the active copies should get corrupted or infected. Note they're in a separate part of the chest from any infected files, so there's no chance of cross-infection there.