The 5Head emote is used when someone says or does something very smart, or pretends to be particularly intelligent. It shows a picture of Cadburry, a popular League Of Legends streamer, with a heavily distorted head.
The 5Head emote is based on the 4Head emote, which also shows Cadburry's head. However, Cadburry's head is so distorted that it looks like he has a huge brain. This makes the 5Head emote reminiscent of "Brain" from "Pinky and Brain."
But what does 5Head mean? Of course, the 5Head meaning always depends on the intention, but mostly it is used to describe a statement or play as intelligent. However, as is common with emotes and memes, it can also mean the exact opposite and be used ironically. In such a case, it expresses that someone is doing or saying something stupid and you want to mock them for it.
The fivehead meme first appeared in 2018. Its popularity increased rapidly in the following months, after which the communities of Cadburry and xQc started using it more regularly. So it's not surprising that the 5Head emote is one of the most popular emotes on Twitch today.