I suppose it depends on context. Could be a physical loop hanging down somewhere, or could be an electrical "loop" of wire tying board grounds together with cabinet grounds.
-shrug-

A loop is when you create multiple paths to ground for the same connector. The loop can pick up unwanted current via induction.

If your coin door is already grounded, don't connect ground braid to it. DO make sure it's properly grounded.

The ground braid is a safety ground, and usually not a part of a circuit ground. A pinball machine can operate fine without the ground braid, it's purpose is to make sure that you don't become a part of the ground path for high voltage in the event of a short circuit.

A ground loop is when there are multiple paths to ground and they are not all at the same potential. Ground loops can be anywhere in the system, not just the same connector Problems such as noise in audio systems are caused by ground loops, as noise immunity in amplifier inputs is compromised. Star grounding is one method to combat ground loops. Star grounding is connecting all circuit grounds to the same point, so that they are all at the same electrical potential.

Every metal part that the player could possibly have contact with at some point, needs to be connected to the ground braid. Legs, rails, lockdown, speaker grills.....

That way, those metal parts are a better path to ground than you are.

Ground loops don't involve induction.

[Safety definition] Maybe in the pinball community there is a tongue and cheek definition of a ground loop referring to the ground braid that loops everywhere in the cabinet to ground all metal parts for safety.

[Noise definition] In the electronics trade, we refer to a ground loop as a multi point ground in interconnected systems that generates unwanted AC noise on audio, video, and other sensitive circuits.

https://en.wikipedia.org/wiki/Ground_loop_(electricity)