This little piece of plastic doesn't look like much but it is the pathway to reducing your swear word tally when sewing. Sadly I don't actually know what it's called or where you buy it from as my mum gave it to me.
What this little sewing buddy does is even out your surface when sewing thick fabric.
I'm sure you've all experienced the needle going up and down on the same spot and the fabric not moving forward when what you're sewing is quite thick. This is because your presser foot is tilted and not able t0 press the fabric down evenly on the feed dogs.
Change the plastic thingy to the correct thickness of your fabric by swivelling the individual rectangular pieces back or forward. Next pop it under the back of your presser foot against the fabric and away you sew, "stuck no more, you will be" (said in best Yoda preview).