I know asc
as a public PGP/GPG key. Not sure how it's used but probably for veryfing a signature of a package somehow.
are text files usually explaining how to install, caveats, user 'manual' etc; the 1st I guess is usually readme.1st and stands for first (readme.first
contains md5sums and you can use it to verify that the content of the file that you have downloaded is not modified since the last generation of the md5sum. You can calculate the checksum yourself and verify against the checksum in the file (e.g. in Windows) and the program md5sum (available in any distro) can read the content and process all files that are listed in the file to verify the checksum.
The only thing I know about meta
is that it stands for metadata; don't know how it's used.