BBT Temping

My 1st month trying this tecnique. The chart on here, what does the cover line mean? My temp was below it to start, dropped & then peaked near cycle day 13 w/ a positive opk too. So my temp has been above the cover line since. I read that a bbt staying high could mean I conceived. Not too hopeful, been taking longer since the MC to conceive again. Even if this is normal & not why it's high, just wanting to know what the cover line means. AF is due between fri-mon. I used to be 25 day cycle, now it's been varying 25-28.