Yes it is a bicycle pant/ankle strap. They help keep your pant leg from getting caught in the chain. They are available a most bicycle stores from $3-$5.
As a third hand brake tool when aligning my brake pads with the rim.
As a spare inner tube strap.
As a parking brake when riding public transportation such as BART. It keeps the bike from rolling around when on the BART train.
Do you have or know of any other uses for this?