@docpop The most popular explanation is "It's like email, but for different social networks", but that assumes that person you talk about understands why email works in the first place.

The most simple explanation to start with is "It's a network of compatible but independent social networks" and "It's also not owned by big corporations". If they are interested, they might ask about "how they are compatible", "why that matters", "are they really independent", etc. And if not, they will probably just say "Oh, cool".