Isekai, Japanese for "Other World", is a popular fantasy genre wherein the protagonist-- typically an average, relatable, everyman loser-- is transported into a new universe. The new universe is often a magical fantasy land, as opposed to something futuristic or space-related, though occasionally the setting will be far-future after the apocalypse, giving it a medieval magic-and-rayguns kinda deal. While it varies series to series, the transplanted protagonist will usually have some kind of ability or powers upon entering the new world, thus allowing them to keep up with the rest of the already-magical fantasy cast and making them a target for the local bad guys.
Methods for being transported are numerous, with body-swapping, misapplied summoning or kidnapping, falling through portals, reincarnation, becoming your video game avatar, getting caught in mysterious storms, hanging out in spirit bathhouses, and getting sucked through magic books being popular.
Note that while "isekai" is specifically an anime/manga term, "portal fantasy" is not, and so many older or western works can be considered to fall in the isekai genre, including things like: A Connecticut Yankee in King Arthur's Court, Chronicles of Narnia, and Alice in Wonderland.