A pond underlay adds extra protection from small stones and roots under a liner. This is highly recommended for every pond liner installation.
The material is a geo-textile fabric, spun, not woven, into the consistency of a fine wool blanket.
Most pond installations feature a layer of sand on the bottom, but the sides are not protected because the sand will not adhere to them. This is where the geo-textile pond liner underlay does the job.
We have heard of carpeting being used for this purpose. This may work well for the bottom of the pond, but to make the folds and transitions necessary in your garden ponds design, specifically designed pond liner underlayment material is a better choice.