Below is a link to an article that outlines the differences in more detail. Basically, they mean the same but "sit down" is more often used as a command (to humans, not to dogs, for example) or to emphasize the verb. "Sit" has a richer meaning, being used metaphorically as well as literally. https://diffsense.com/diff/sit/sit%20down

to sit - to place oneself in a seat or chair in general. to sit down - to place oneself in a seat or chair, after standing up.