What does the number mean in 'ls(1)', 'stat(3)' and 'apt(8)'?

Question

While browsing through the web and local documents, I often see expressions like this:

See apt(8) for more information about the available commands.

My question is, what does the number here stand for?

Answer 1

From the "man man":

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.