.deb package is retrieved to be installed (either as a new package or to upgrade the current version), it is stored in
/var/cache/apt/archives. These packages are not needed after installation, but if you keep them, and you need to reinstall a package, then it doesn't have to be retrieved again.
You can delete these files manually but there is very little reason to do so, as there are better, automatic ways to do it.
If you want to delete all of them, run:
sudo apt-get clean
If you want to delete just the ones that are quite unlikely to be needed again, run:
sudo apt-get autoclean
autoclean does is to delete just the ones that are no longer provided for download. These versions are, generally speaking, sufficiently old and not recommended for use, that you usually won't want to reinstall to them.
Since the effect of
autoclean depends on what packages are available for installation, you may want to update your local information about that first:
sudo apt-get update
If you want to delete specific ones, then it would make sense to manually remove the ones you don't want. You can do that with this method, though if you're comfortable with the command line then this would probably be faster, easier, and (since you don't have to worry about a
root Nautilus window you might forget to close, or in which you might make a mistake with the mouse) less daunting:
sudo rm file1 file2 ...