From Minecraft Wiki
The icon for the Levitation effect.

Levitation is a neutral (sometimes negative) Status Effect that causes a player to float in the air for a short period of time depending on the duration of the effect. When the effect wears off, the player will plummet to the ground which can kill a player if they are high enough off the ground. This effect was added in 1.9.

Like other status effects, it can be removed by drinking Milk.

The Levitation effects can be reversed by being underwater. That is, if you are underwater with the levitation effect, you will sink.

You can get this effect by using commands or getting hit by a Shulkers.