The phrase "sleep surface" refers to the area or material where you lie down to sleep, such as a bed, mattress, or any other surface that is comfortable for sleeping. Full definition
Set up a bassinet, crib, or other safe sleeping surface for your child in another room, to allow you use of your bedroom during naps, or in the early evening. (attachmentparenting.org)
A more soft sleeping surface can be a threat to the kid because it makes the baby sink in. (justbabybeds.com)
All babies should be put to sleep on a flat, firm sleeping surface with fitted sheets and no blankets, loose clothing, or anything else soft in the sleeping area. (verywellfamily.com)