“isekai” is “another world”.

The protagonist ends up leaving our world and is transported/reincarnated in another – usually JRPG-like medieval fantasy themed – early in the show. The stereotypical mechanism is that the protagonist gets run over by a truck… Usually they end up in some sort of power fantasy story taking advantage of either their knowledge/technology from our world or special skills they got via the reincarnation or whatever.

There’s, uh, a lot of them… of varying quality.

Some popular examples are Konosuba, Re-Zero, and Mushoku Tensei. Which shows qualify as trash is, of course, a matter of taste. 🙃️