It's the section of the manual. Compare e.g. "man 1 write", documenting the utility /usr/bin/write, versus "man 2 write", documenting the C function write() in <unistd.h>.
"man man" for a section listing:
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), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]