Luteal Phase is the phase between ovulation and your next period. If you are charting, Luteal Phase is commonly counted as DPO (Days Past Ovulation). For example, if you read someone's chart stating CD23, 8DPO, it means this person is on her 23rd day in the current cycle, and is 8 days past ovulation, so you can assume she must have ovulated on CD15.
Luteal Phase is a vey important phase if you are TTCing. It is important in 2 ways. It is important for you to determine when you are about to ovulate, and it is important for an impregnated egg to implant into the lining of uterus.
In a textbook period, the length of the cycle is 28 days, ovulation happens half way through the cycle, the LP is 14 days. However most women don't have the textbook period. Luteal Phase varies from woman to woman. Some may have a short LP, as short as 8 days. Some may have a long one, as long as 21 days. When LP is shorter than 10 days, you will most likely experience fertility problems.
The good news is the length of LP stays the same for one woman, it doesn't change from period to period. So when you establish the length of your own LP, you are one step closer to a successful cycle.
please read the topic "How does a short Luteal Phase affact you?" and "How does a long Luteal Phase affect you?" for more details.