Part time weeblet here

Isekai if I remember right translates to other world. However in the context of the genre it’s when the protagonist gets transported to what is usually a fantasy world from what is usually death or reincarnation and sometimes summoning

Another trope I’ve seen is protagonist just wakes up in an MMO as their character and all the NPCs are sentient

I’m sure there are more but in anime it mostly boils down to: character is in fantasy now and knowledge of our world could give them an edge

When done right and done good it’s also a great trope for a fresh start and world building cus we don’t know the world, and the protagonist doesn’t know the world so we learn with them.