Negative opk CD 15

I know this is a dumb question but what does "CD15" mean???

 

cycle day 15

 

Ohh sounds so simple now lol, thanks.

 

i put chart day 15 1st

 

Ok thick question Prob, but i know nothing about OPKs
but i can see 2 lines, i would have thought that meant + like with a PG test

Please explain slowly for me LOL

 

Jo both lines have to be exactly the same colour or the one on the left has to be darker!!
xx

 

Ahhh righto