If you look at source2e you might see a lot of lines that look like
\foo a one-argument command that has regular and starred versions. The starred version is the expansion of
\@foo while the nonstarred version is that of
\@@foo. Using the @ sign in the auxiliary macros is a TeX convention which some authors embrace and some avoid.
There are higher-level ways to do it (as lockstep points out) but once you learn this pattern it's not too hard to use. Just make sure it's between
\makeatletter...\makeatother or in a
Edits removed some inaccuracies and editorializing.