It's the section that the man page for the command is assigned to.
These are split as
- General commands
- System calls
- C library functions
- Special files (usually devices, those found in /dev) and drivers
- File formats and conventions
- Games and screensavers
- Miscellanea
- System administration commands and daemons
Original descriptions of each section can be seen in the Unix Programmer's Manual (page ii).