The term is not coined by RingCentral. It's been in the telephony field for ages. Google "call leg" or "agent leg" and look at images. Also look up "call structure"

I am copy pasting some info for you in the context of Contact Center.
Agent Leg = Connects the Contact Center to the correct RingCentral device, so that Contact Center call can be interacted with.
Contact Center = Central point where all customer interactions are managed across various channels, where contacts are not assigned to a specific agent, but rather a skill, which is handled by a team of agents.

By the way I don't have a deep understanding of this, so this is the best I can explain it. Don't think it's that important?

Not really, I just had a really weird itch to know what it meant