It searches for a keyword in the manpages and tells you what page contains the text.

    $ apropos printf
    fmtcheck(3)              - sanitizes user-supplied printf(3)-style format string
    format(ntcl)             - Format a string in the style of sprintf
    fwkpfv(1)                - FireWire kprintf viewer
    fwprintf(3), swprintf(3), vfwprintf(3), vswprintf(3), vwprintf(3), wprintf(3) - formatted wide character output conversion
    printf(1)                - formatted output
    printf(3), fprintf(3), sprintf(3), snprintf(3), asprintf(3), dprintf(3), vprintf(3), vfprintf(3), vsprintf(3), vsnprintf(3), vasprintf(3), vdprintf(3) - formatted output conversion
    wprintf_l(3), fwprintf_l(3), swprintf_l(3), vwprintf_l(3), vfwprintf_l(3), vswprintf_l(3) - formatted wide character output conversion