Below a video simply shows how geogrid works.
Ofcourse there are a lot of things to do for design and applications but this video simply explains the supporting power of geogrids.
Take a box and fill it with gravel.
Somebody step on gravel.
The gravel does not hold the person up.
Then reinforce the gravel with geogrid layers.
Next when you remove the box, you can see that system is supported by the geogrid layers.