Search the whatis database for strings.
Syntax apropos keyword ...
The whatis database is a set of database files containing short descriptions of system commands for keywords. apropos searches this database and displays the result on standard output.
List all the man(1) and man(8) commands
$ apropos ' ' | egrep '\(1\)|\(8\)'
“A man travels the world in search of what he needs and returns home to find it” - George Moore
Related macOS commands:
whatis - search the whatis database for complete words.