It's easy to understand why you offset the y value-to give you spacing between the lines. Imagine a brick wall. Each successive line of bricks is offset to the right a little to create a pattern. So a .5,1 offset would space the bricks upward by one unit and every other line would offset .5 to the right of the origin, creating a stepladder effect. It's the same pattern used for creating line types.
All positive numbers represent the number of unit spacings that the pen is held down.
A 0 represents a dot. The negative numbers represent the number of unit spacings that the pen is lifted up. Therefore, it produces a pattern of dash, dot, dot, dash, dot, dot and so forth. You will see the complete path specifications for your pattern and the file name. Pick the file name, and you will see a preview of your pattern, as shown in Figure 2.