What does the name of the unix command apropos mean

The stuff in there is largely Unix-idiom (chown, fork, gethostname, nice), so I'm guessing that it originally did mean Unix. It's part of the POSIX standard, though, so it's no longer just Unix.

man displays man pages using a pager - in your case, less. When you press h, you're seeing less's help.

So, as for what "spanning files" means, it's referring to searching across multiple files simultaneously opened in less.

It doesn't apply to the man page use case (since there's only one file opened at the time), but if you did

less file1 file2

at the command line, and then searched for a pattern that occurs in both files with /foo, you could jump between occurrences of the pattern inside file1 using n repeatedly, but it would eventually stop when you hit the last occurrence in the file. At that point, if you were to hit Escn (or usually also Alt+n) it would jump you to the first occurrence of that pattern in file2.