A user guide, also called a user manual or (less frequently) instruction manual, is a book about how to use something.
A computer usually has a user guide to tell people how to use it. A computer program also often has a user guide.
The user guide is often written by the person who made the software.
People who want to write a user guide must keep the words very simple so other people can understand what you are saying. It also helps to put many pictures into the user guide.