Black fleece back warmer,  soft and flexible fabric with elastic.
 
Helps to warm up the muscles in your back, and keep them warm during different elements of training. Protective layer to the lower back when rolling. 
 
Could help prevent injury by maintaining muscle warmth.
 
Worn by many gymnasts during drill work or ballet sessions, to maintain muscle temperature in the back. 
Also cushioning for the lower back when rolling.