Hi Peter! It's a piece of velcro! It's how I attach 2S battery packs to my radio for field use.
I use 2S 18650 holders for FPV goggles, they happen to have the same plug, and come with built in chargers sometimes.
The official velcro brand is extremely strong once the glue sets (24H) and i've never had the strips of velcro come off when removing a battery.
