Q: How do I work metal fastener on strap?

Question

This is on a medical device to protect me One end of cotton strap is attached to middle of clasp that has little moveable toothy thing. You've seen 'em.
I need to be able to pull end of strap through it so it cinchs and STAYS TIGHT
Make it simple please

Answer 1

It is a buckle.
The strap passes under a bar, over the "toothy thing", and under another bar.
When it's pulled tight the 'toothy thing" will grip the strap if you've done it right.