Isekai anime, also known as person in a strange world, is a genre of anime in which the protagonist finds himself / herself / themselves in a different and strange world.
Some famous titles include: No Game No Life, In Another World with my Smartphone, Konosuba, Overlord, and Sword Art Online(even though some say it is not and put it in the mmorpg genre) to name a few.
The main plot of this genre of anime is that the protagonist, often portrayed as a shut in neet who is unsatisfied with the world, or in some cases are living a good life, are suddenly transported into another world or parrallel universe, either through a premature death, a portal opening, or a summons from that other world.
Personally, this is one of my favorite genres of anime. Often times the protagonists are overpowered in this new world, attract a harem, and go on great adventures. These other worlds or parallel universes often resemble worlds from a video game, particularly an mmorpg and have many fantasy elements like elves, dragons, and often magic. The protagonist is often the hero, or chosen one in these worlds, whether by being summoned or recognized as a hero through their overpowered strength.
Many critics of Isekai anime point out how many stories use the same tropes over and over, often with differing variations and details that unfortunately bloats the genre and makes it redundant.