Underlay is the layer beneath a carpet that you don’t see but definitely feel. A good, soft underlay makes the carpet feel thicker, more comfortable and more luxurious underfoot. Walking becomes quieter and gentler on joints.
It also helps the carpet wear more evenly, because the underlay absorbs some of the pressure from footsteps. That means the carpet itself can last longer before flattening or looking tired.
Underlay can also add a bit of insulation, helping with warmth and slightly reducing sound transmission between floors.
People sometimes try to save money by skipping or skimping on underlay, but often the quality of the underlay has more impact on everyday comfort than the difference between two similar carpet styles.