To scribble is to write something quickly and sloppily. You can also call someone's handwriting, if it's hard to read, a scribble.
Have you ever made a note of something very quickly, and then later when you tried to read it, you couldn't? That's a scribble. People scribble when they need to write fast. The problem with scribbling—-making a scribble-—is that it’s hard to read. Some people have handwriting that is always hard to read: you can call poor handwriting a scribble. Doctors are famous for their scribbles. The Latin root is scribere, "to write."