What does "open world Mario Kart" even mean?

So many people are sharing their hopes that the Mario Kart teaser is actually for an open world game but how does that even work? All the courses are on one map and you just drive off-road from one to the next? What about tracks in space and volcanoes and stuff, how would you even drive to get to those places? What would even fill the spaces between courses? Roads? Grass? Cities? Random NPC's also in karts? Does it work like Zelda with huge empty space with nothing meaningful to fill it? Does it work like GTA where you can leave the kart and walk around? Is it more like Mario Odyssey but you drive and hop in the kart instead? The math just isn't mathing for me.

EDIT: Thank you to everyone who taught me what this means! My hopes aren't high that Nintendo will do it, but the prospect is exciting!