It's an ejection port cover. it's the ejection port cover. - Luth-AR Ejection Port Cover Assembly

It's a plain jane, "milspec" ejection port cover for 5.56 AR-15 uppers at a fair price. Handily, it comes with the c-clip already attached to the rod, which saves you what can be a tricky step for newbies. Just make sure the spring is properly tensioned, insert the rod, and you're ready to slap on your barrel nut. If you're smart enough to use an AR-15, you're smart enough to install this cover.

I don't know why Magpul, Strike, Forward Control, and others think they need to "improve" what has been a simple and effective design for the better part of a century. Skip the red anodized hypebeast parts and go with what works.