When you jot something, you write it quickly. You might jot down a friend's email address on the back of your grocery list.
It's most common to use the verb jot with down: "Hang on, let me jot down the appointment time." It's a good word to use when you're writing a brief note, a phone number, or a list — especially when you're doing it in a hurry. The earliest meaning of jot is "a tiny bit," which comes from the Latin word jota, an alternate spelling of the Greek iota, the smallest letter in the alphabet or the smallest amount of something.