Luteal phase (LP) is the time from ovulation to menstruation.
If you ovulated on day 20 and your period started on day 36 - then your LP is 16. The length is important because a shorten LP could cause either infertility or an early miscarriage. Some doctors say a shorten LP is 10 days or shorter, some say 12 days or shorter. Mine is 9 days
There are things you can do to length your LP. I started taking B6. I don't know if it is working yet since I'm on day 11 of my cycle and I normally ovulate on day 18.