In this video I remove a copper olive from some 15mm copper pipe buy cutting the olive with a junior hacksaw. You can often remove olives without cutting them, and you can even get special tools for pulling or cutting the olives, but these can be expensive. In the UK we call them olives, they are normally copper or brass, in other countries they call them ferrules and are used in every compression joint.
How to remove an olive- https://www.youtube.com/watch?v=CjvDKvPJlmE&t
How compression fittings work- https://www.youtube.com/watch?v=KOxZE4zL6rE&t