ELI5 stands for, “Explain Like I’m 5.” The 5 in the phrase refers to a 5-year old kid, which suggests the over-simplification of an otherwise complex concept so that the person inquiring would have a grasp of its meaning despite his or her limited understanding (or the lack thereof) of it.
Explain Like I’m 5 was derived from the subreddit r/explainlikeimfive. This subreddit was created by a Reddit user, boss galaga back in September 2011. The term “ELI5” was integrated into the Urban Dictionary a month after.
The phrase went on to become one of Reddit’s most popular boards. By 2019, r/explainlikeimfive ranked as the 17th most popular subreddit on the platform. At present, it has over 16 million subscribers.