"Include what you use" means this: for every symbol (type, function,
variable, or macro) that you use in foo.cc (or foo.cpp), either foo.cc
or foo.h should include a .h file that exports the declaration of that
symbol. (Similarly, for foo_test.cc, either foo_test.cc or foo.h should
do the including.) Obviously symbols defined in foo.cc itself are
excluded from this requirement.

This puts us in a state where every file includes the headers it needs
to declare the symbols that it uses. When every file includes what it
uses, then it is possible to edit any file and remove unused headers,
without fear of accidentally breaking the upwards dependencies of
that file. It also becomes easy to automatically track and update
dependencies in the source code.

WWW: https://include-what-you-use.org