They describe the section of the manual, where 1 = programs, 2 = syscalls, 3 = userspace functions, 4 = special files, 5 = file formats, 6 = games, 7 = misc/overviews.

So printf(1) is the man page for the /usr/bin/printf command, while printf(3) is the man page for the libc printf() function.

Alternatively, readdir(2) is the man page for the readdir syscall, while readdir(3) is the man page for the libc wrapper, which no longer actually calls readdir(2). See also syslog(2) and syslog(3).

Or, time(1) is the man page for /usr/bin/time to time how long commands take. time(2) is the syscall to return the number of seconds since the epoch. And time(7) gives you an overview of time and timers on the system.

Unless you're on a UNIX system the number isn't important. If you are on a UNIX system, then it's useful for telling the difference between commands (like write(1), hostname(1), or printf(1)), system calls (like write(2)), library functions (like printf(3)), or config files (like hostname(5)).