Although others have explained things a bit, I'll try making an analogy to win32: Section 2 ("system calls") contains functions which are traditionally serviced

Although others have explained things a bit, I'll try making an analogy to win32: Section 2 ("system calls") contains functions which are traditionally serviced directly by the kernel. In win32, this would mean that they (historically) correspond more-or-less directly to a Nt*() function in ntdll (or a kernel entry point). Section 3 ("library functions") are all other library functions, including ones serviced by third-party libraries.