The numbers signify what section the page belongs to, there are 8:
1 - General commands
2 - System calls
3 - C library functions
4 - Special files (usually devices, those found in /dev) and drivers
5 - File formats and conventions
6 - Games and screensavers
7 - Miscellanea
8 - System administration commands and daemons
for example, if you wanted to know how to use the
crontab command, you could type man crontab. If you wanted to know the layout of the crontab file, you'd use
man 5 crontab.
Not all items have all 8 pages. If the specific item is not a command, it will automatically direct you to the appropriate page as there was to entry in section 1 for the command. An example of this is
man strcpy which will bring you to the C library function strcpy.