"Retrosternal" means behind the sternum (breastbone), while "fat pad" refers to a localized accumulation of fat tissue. Therefore, a retrosternal fat pad is a collection of fat located behind the sternum in the chest area.

the first function of a fat pad is cushioning and the second is shock absorption. You can find them in Synovial joints.

a fat pad

Hoffa's pad, also referred to as the infrapatellar fat pad is located below the kneecap. This soft tissue separates the kneecap from the end of the thigh bone called the femoral condyle. When the kneecap is hit, the infrapatellar fat pad works similar to a shock absorber, protecting the structures underneath.

The fat pad is a normal structure in the knee. It is literally composed of fat and is bright yellow in colour and exists below the patella (kneecap) just behind the patellar tendon. In this position it buffers any shocks to the front of the knee, but itself commonly suffers injury and problems related to the fat pad are not uncommon.