man pages

Those numbers represent the section of the man pages:

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 eg /etc/passwd
6. Games
7. Miscellaneous (including macro packages and conventions)
8. System administration commands (usually only for root)
9. Kernel routines [Non standard]