Seam taping is a process that ensures the effectiveness of a pants waterproofing. This is achieved by taping, with waterproof tape, behind any stitching on the pant. This eliminates the ability for the stitching to leak.
Critically Taped pants are typically taped on the rear seams. The means the pants are reinforced in the areas you will be sitting on.
Fully Taped pants have every stitched seam on the pants taped. This means that the pants will not leak anywhere that the fabric has been stitched or pierced by a needle.
Pants with None means that no seams have been taped. These pants are more reasonable in price, but can be prone to leaking at the seams.