What does it mean when you dream about a snake?

it means that an evil person is trying to scare you by trying to be a snake

well a snake symbolises evil, so yea

More information:

Although snakes may represent different things in different cultures and to different individuals, the most common representation is as something evil and untrustworthy. In Western Culture one speaks of an evil, plotting scoundrel as "a snake in the grass."

... However, an entirely different connotation is given to the image of a snake raised on a stick, tree or rod, in which case the snake represents healing and the medical professions. Similarly, a snake forming a circle or figure 8 with its tail in its mouth represents regeneration or eternal life.

.... So the meaning of a snake dream must be considered according to the position and action of the snake, as well as its cultural context. The most important factor is always the meaning snakes have in the mind of the individual dreamer.

It means you was previously thinking or saw something to do with snakes or related in figure of shape therefore you've dreamt about it.