Read this:
http://www.subsim.com/radioroom/showthread.php?t=92743
to interpret the lines made on the example pictures, just remeber that they represent aproximate positions for the target at different points in time that where given to you as intervals (for example "medium range") and hence they're not points but lines that go from "here to there" according to the range estimate given. You can see that as the range to target closes the lines shift to smaller sizes. For example the last ones are only 1 km long, since "short" range means from 0km to 1km, whereas the previous ones where 2km long, because "medium" range means more than 1km but less than 3km (giving you a 2km interval). And of course, the big line that represent's the target's track is drawn and continualy updated so that it cuts through all of the "contact" lines (this assumes that the target is traveling in a straight line).
Anyway, just read it carefully and I'm sure you'll get it.
Also, observe how this technique is used auxiliarly in the 2nd part of the "Video Hunt":
http://www.subsim.com/radioroom/showthread.php?t=97682
And of course, there's allways this thread:
http://www.subsim.com/radioroom/showthread.php?t=88961