It is the informal second person singular imperative form of the verb "s'asseoir" (to sit down).
Here's a breakdown:
* Assieds: This is the imperative form of the verb "s'asseoir" in the second person singular.
* -toi: This is the pronoun "toi" (you) attached to the verb, indicating that the command is directed at a single person.
Therefore, "Assieds-toi" is a direct command for someone to sit down.