The meaning of the numbers are explained on the page you linked to. They refer to coordinates relative to the size of the image. Let's say your image is 500px tall and 1000px wide. Now imaging placing a grid over the image such that each pixel is one unit. The origin, coordinate (0,0), is the upper-left corner if the image. So if you want the pixel 25 rows down and 65 columns across, the coordinate would be (65,25).
The extension offers 3 different shapes: rectangles, circles, and generic polygons. There are 4 numbers for a rectangle. In order:
- x-coordinate of the rectangle's top-left corner
- y-coordinate of the rectangle's top-left corner
- x-coordinate of the rectangle's bottom-right corner
- y-coordinate of the rectangle's bottom-right corner
For a circle, there are 3 numbers. In order:
- x-coordinate of the circle's center
- y-coordinate of the circle's center
- radius of the circle
For a generic polygon, the numbers are, in order, the coordinates of each of the polygon's vertices. Here is an example of a rectangle and a generic polygon that correspond to the same region in an image.
rect 25 60 75 100
poly 25 60 75 60 75 100 25 100
The shapes will scale with the image. This way, you can change the display size of the image without changing all the coordinates.
If you want a graphical way to make the image map, take a look at WikiHow.
Edit:
I just checked WikiHow in detail and the instructions don't quite tell you how to get to the map editor. I did some digging and found this site listed on the documentation page you linked to earlier. It also appears to be the site pictured in step #4 on WikiHow. Rather than uploading your image to WikiHow, you should upload it to the relevant wiki. Get the URL of the image and use it as the image input for the map editor.