What’s this part called? Looks like it’s come as part of a kit, along with the quick-release skewer. Would like to buy a replacement but not sure exactly what i’m looking for!

That’s a single speed screw style tensioner.

https://www.amazon.com/Origin8-Chain-Tension-Adjuster-Black/dp/B001GPJY1O/ref=asc_df_B001GPJY1O/?tag=hyprod-20&linkCode=df0&hvadid=692875362841&hvpos=&hvnetw=g&hvrand=5697858012028321115&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=1014465&hvtargid=pla-2281435178298&psc=1&mcid=ecf5ab800cd63cbe9b80f2ba4d477402&hvocijid=5697858012028321115-B001GPJY1O-&hvexpln=73&gad_source=1

Basically, the axle threads through the inserts that sit in your dropouts and you tighten or loosen the screw at the back to tension your chain.