add-apt-repository
PackagesPackage: software-properties-common
Adds a package source (usually a PPA) and its key. Every source added is a third party you trust for everything it publishes; the list should stay short.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
add-apt-repository --remove- Removes a previously added repository. Packages already installed from it stay where they are — removing those is a separate decision.
Lessons that teach it
No lesson names it among its useful commands; it appears inside other lessons' command lines.