While browsing through several pages of nlab(mainly on n-Categories), I encountered the notion "foo" several times. However, there seems to be article on nlab about this notion. Is this some kind of category theorist slang? Please explain to me what this term means.
It's slang, which I've mostly seen used in the context of computing rather than category theory; foo is just a placeholder for something else, as is bar. A logician I know likes talking about widgets and wombats $-$ it all serves the same purpose.
For example, you might say "an irreducible foo is a foo with no proper sub-foos".