What are slipper socks?

Slipper socks are socks are worn on the feet, with anti-slip gripping tread on the sole of the foot to prevent falling on slippery surfaces. UK hospitals provide these to patients who stay in the hospital overnight. Some have the grippy tread on both the sole and the top of the foot. This is so that even if the socks twist around when walking, the patient is less likely to fall.