This is a ring sizer belt to correctly measure your finger.
Make sure you use the correct finger on the correct hand. 
Put the end of the ring sizer through the “buckle”. Slowly pull it  through until you have a large sized ring that you can easily put onto  the finger. Put the ring sizer over the chosen finger and gently pull it tighter until you have the correct size. 
The correct size should allow the ring sizer to move over the knuckle without too much effort.
At that point, read the size indicated and select it while purchasing.
The ring sizer is reusable as many times as you want.
It costs € 5.00