This is an affirmative answer which would mean you should nod your head for the whole statement. Since it’s a response, you typically would raise your eyebrows as a way to add clarity to the fact that you do indeed understand.
You will notice that the UNDERSTAND sign isn't signed right by the forehead like in the vocabulary video. This is how one would normally sign UNDERSTAND in a sentence - the UNDERSTAND by the forehead is a more formal way of signing.
