It's the period of time between ovulation and your period. On average it should be 12 to 16 days. Anything shorter than 10 days can be a problem as far as conceiving, and 10-12 days is a grey area where some people may have issues and some doctors may treat at that point.
You figure it out by figuring out when you ovulate and then counting the days between ovulation and the start of your next period. There are a few ways to figure out when you ovulate- ovulation predictors can work, and charting your cycles and natural fertility signs is probably even better.
There are 4 parts to your cycle: menstruation (your period--when hormones cause you to shed your uterine lining if no fertilized egg has implanted), follicular phase (from when your period starts to when you ovulate--when hormones stimulate follicles to form), ovulation (when hormones stimulate at least one follicle to rupture and produce and egg), and luteal phase (from ovulation until your next period starts--when hormones stimulate your uterine lining to thicken in preparation for a fertilized egg to implant).