A group is a shared identity bucket that helps manage access for multiple users or services.
If group meaning stays fuzzy, permission models feel more complicated than they really are.
A group lets the system apply one set of rules to many users or accounts at once.
A group is a team label the computer can use for shared rules.
A group is a way to manage shared access for more than one identity at once.
When people only think one user at a time, group-based access can feel abstract. It is really a way to avoid repetitive permission work.
Study the larger concept after the word itself becomes clear.