Isekai is a sub-genre of Japanese Fantasy. In recent years, it has gotten incredibly popular. So many Isekai stories have been released that it’s hard to know what it even is.
The core element of this sub-genre is the idea of coming from our world and going to another world, usually a fantasy world. This is usually done through reincarnation after dying. For some reason, death is usually the result of being hit by a truck. This is so common it’s become an in-joke among fans of the genre and Anime.
Isekai is so (relatively) old now that it’s basically impossible to describe what these stories have in common.
For example, the Villainess sub-genre likely originated in isekai, but has little in common with other isekai outside of reincarnation. Villainess isekai typically has the protagonist reincarnate into a fiction story, such as a novel or game. Whereas most isekai has the protagonist reincarnate into a fantasy world, usually not a work of in-universe fiction. There are exceptions in both sub-genres of course.
It’s hard to describe the sub-genre, due to the insane variety.