The reason they are asking if the carpet has a high or low pile (Pile = how thick the yarn is from where the yarn is fastened to the carpet backing) is because on a good grade of chair mat, the underside of the mat has a grid of protrusions (like rows of small needles about 1 to 1 1/2 inches left to right and north to south).
That prevents the mat from crushing the carpet it is resting on. These "needles" or "pins" are usually made of the same material as the mat (hard, heavy-wear plastic). On a carpet that had a very high pile (1/2" to 3/4" or more) plus padding (of 3/8" to 1/2" thick), you would want to order a chair mat with a grid of longer protrusions. This would keep the chair mat from crushing the carpet completely flat...bad news if it was crushed completely flat and you decided to rearrange your furniture.
See if your description gives you more details, like how low or high the grid of protrusions are on the underside of the mat. The protrusions have two purposes (1) to protect the carpet from being crushed (2) to prevent the mat from slipping. A very low pile, commercial style carpets without padding, you would choose the shortest protrusions possible.
I think they call the protrusions "cleats", but I'm not sure.