If I knew what it was I probably still wouldn't have an idea lol xxxxxxxxxxx
It's the number of days between ovulation and your next period. It's usually 12-16 days with an average of 14 but it's pretty much constant for each person (unless you're awkward and breastfeeding like me!)
If it's too short (some doctors consider 12 short, but generally 10 or less) then implantation may not occur before the lining of your uterus starts to shed. Implantation is usually 6-10dpo though, so even with a 10 day LP there's still hope x