known by several names (ADL, as above, 'quick-away' and 'one-way' layers/liners), they're simply a loose matting or floss of material which disperses fluid and it's energy, thereby helping to keep the fluid in the absorbant pad.
a case of convergent technology, i invented such in the 80s, along with the 'big boys', but before them, and their first use was with terry-towelling nappies.
i still use them regularly and i am using one now, in the form of a pillow protector turned inside-out (yep, works out cheaper than buying the specific material alone from the haberdasher).
if you're interested in the principles at work, you can see them by exposing a humble plastic or metal pan-scrub to the full flow of tap/faucet water.