Coverline????!

So Ive just gotten a cover line on my chart. First time as last month I didn't chart properly kept missing days here and there.

What am I using this cover line for? I've searched google but their answers are but in such complex terms so can anyone explain in a basic how and what I look for in regards to this cover line?

Also do you think glow has estimated my O correctly

The opk I used showed a much darker test line than the control so would of thought that meant I was super close to O but again not too sure

Thanks ☺