The numbers refer to the manpage section the manpage belongs to:
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
7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
man man :-) )
The reason for showing the section is mainly that there is often a manpage with the same name in more than one section, e.g.
man command), and
man(7) (the system of manual pages). Same for e.g. system calls (section 2,3) and commandline programs (section 1) with the same name, e.g.
BTW, you can use
man -a to see man pages from all sections for a given search term.