A slat is a strip of wood or other thin material. The blinds that cover your windows are made up of slats.
The horizontal slats in Persian or Venetian blinds are often made of plastic, fabric, or metal, and they can be adjusted to let light in or shut it out completely. Some wooden fences are made of slats as well, and bed frames often have slats that support the mattress placed on top of them. Slat was originally sclat, from the Old French esclat, "split, chip, or piece."