It's the "man" section. According to "man man", 2 is the section for system calls. Often you'll have manpages for the same name under different sections, for ex

It's the "man" section. According to "man man", 2 is the section for system calls. Often you'll have manpages for the same name under different sections, for example sync(1) is a command while sync(2) a system call (which, unsurprisingly, the sync command uses).