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

If I was saying, "I'm going to sit..." then it depends on whether anything comes after "sit". If I am talking about the place where I will sit, then I don't need to use "down". For example: "I'm going to sit in the sofa for a while." "He sat on the floor because there weren't any chairs." "We sat next to each other on the train." * Note that you could use "down" for these but in that case it emphasizes more the motion of sitting and less the location. But if I don't mention the place, then it becomes more common to use "down". For example: "I'm tired and I have to sit down." "He walked into the room and sat down without a word." "I have to tell you something important. You'd better sit down for this." * Note that "down" isn't absolutely essential for these but I think it sounds a bit more natural.

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.