Body doubling is when you do something with someone else, without direct interaction. For example, both of you working on your laptops in the same room, but you're not talking to one another much, or directly helping one another, just aware what the other person is doing.
You might be doing different things (one person fixing something, one person cleaning something).
It helps because you're both creating a sense of "this is a place and time where things get done" but you don't have to spend any energy worrying about social interaction, whether you're saying the right thing, or your work being judged.
It helps with Autistic and ADHD inertia, when it can be really difficult to just start working, and to stay in the zone.
Another person just being there, doing things, makes it easier to start working too.