A role is a collection of permission. For example the Teacher role has capability mod/quiz:preview, and does not have mod/quiz:attempt.
Normally, these capabilities are set globally, so the Teacher role can do the same things everywhere.
However, sometimes, you want to change the definition of a role in a particular place. For example, in one particular Forum, you might want to give students the ability to moderate that forum. You can do this with role overrides. Override the Student role in that forum, to allow it a few extra mod/forum:xxx capabilities.
I hope that makes it clear.
Roles and permissions
Confused between "Assign Roles" and "Override Roles"
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