Spiders web navigation is where you use a fixed point on a chart, normally the easiest is to use a compass rose, nearest to the area that you are going to be operating in.
Draw lines from the centre of the compass rose at 10 degree intervals. Then using either a pair of dividers with a pencil attached to them or a compass, measure extending distances in half mile intervals from the centre of the compass rose. Looks a bit like this: NOTE: none of the bearings or distances are to scale, this is just to explain the concept.
This is only really useful if you can put the position of the compass rose into the GPS as a waypoint and then make the GPS goto it. You will then be given a bearing and a distance, with this you can then plot it onto the chart and instantly work out where you are. For example you could be where the red cross is and instantly work out where you are.
Hence called a spiders web as it looks a bit like one.