I’m always forgetting what they mean and how to use them.

There’s a great answer at: https://unix.stackexchange.com/a/3587/223606

For me, the important parts are:

MANUAL SECTIONS
    The standard sections of the manual include:

    1      User Commands
    2      System Calls
    3      C Library Functions
    4      Devices and Special Files
    5      File Formats and Conventions
    6      Games et. al.
    7      Miscellanea
    8      System Administration tools and Daemons

    Distributions customize the manual section to their specifics,
    which often include additional sections.

And the syntax:

$ man 1 printf
$ man 3 printf
$ man -a printf