Definition: A unit vector is a vector of length 1
i.e. |U| = 1. (|U| is its length.)
Suppose a = (xa, ya) and b = (xb, yb). The vector shown here is set up so that (xb - xa)2 + (yb - ya)2 = 1, i.e. U has length one.
In the sketch, you can check it out by moving a to your favorite points and then see what you can choose for b; when you find a nice vector you can add its coordinates to the table by double-clicking on it.