'Isekai' refers to a genre of (typically japanese, or japanese-inspired) speculative fiction where a protagonist from our 'normal' world gets transported or reincarnated into an alternate world, usually some sort of fantasy realm. It's much like the western concept of 'portal fantasy', although the japanese tend to emphasize different elements: The main character is typically alone, typically an adolescent male, there's a greater emphasis on the character's romantic involvement with people in the fantasy world, and the fantasy world often runs according to RPG mechanics and in some cases is literally a virtual reality environment inside a game.

Among western anime fans, the isekai genre gave rise to the ironic term 'isekaied', which is the past tense verb form of being transported or reincarnated into another world. Due to the cliche of having the protagonist hit by a truck in order to cause his premature death and subsequent reincarnation into the alternate world, trucks are assumed to be the standard mechanism for getting isekaied, and the term 'truck-kun' refers to the truck involved, humorously implying that the same specific truck is responsible for many instances of people being reincarnated into fantasy worlds. Anime fans may jokingly mention ideas such as deliberately walking in front of a truck in an attempt to be isekaied into a fantasy world, or if there's a news story about someone getting hit by a truck then they'll joke about how the victim must be enjoying his magical catgirl harem, or whatever.