For example, `man 2 select` (or `man select.2`) gives you information about the `select` syscall, and `man 3 select` has information on the libc wrapper around that syscall.

A somewhat better example might be time: `time.1` has info on the command use to calculate running time of another command (in statements like `time make -j`, and `time.7` — low-level information on how to interact with kernel's time-related functionality. Since both are named `time`, if you just use `man time` without the number, you get the first one (and no way to get to the second).

Read `man man`, it has a lot more on this stuff.

  The table below shows the section numbers of the manual followed by the types of pages they contain.
  
         1   Executable programs or shell commands
         2   System calls (functions provided by the kernel)
         3   Library calls (functions within program libraries)
         4   Special files (usually found in /dev)
         5   File formats and conventions, e.g. /etc/passwd
         6   Games
         7   Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7), man-pages(7)
         8   System administration commands (usually only for root)
         9   Kernel routines [Non standard]