In output from `apt update` what do 'InRelease' and 'Release' refer to?

InRelease files are equivalent to Release files with the exception that they contain an inline GPG signature, whereas validating Release files requires downloading a separate Release.gpg file. Having the signature in line avoids race conditions when downloading.

This FTP master meeting summary contains this feature’s announcement, with a brief description.

Since the title asks about them too, I’ll mention that Release files contain distribution metadata and the checksums of the index files.

Another reference to this is here:

InRelease files are signed in-line while Release files should have an accompanying Release.gpg file.

https://wiki.debian.org/DebianRepository/Format#Origin