The luteal phase is the second phase of the cycle– after ovulation and before menstruation. This phase ends when the period/menstruation begin.
After ovulation, progesterone is produced from the remains of the follicle that released the egg, the follicle is now called corpus luteum.
Corpus luteum is what produces progesterone, which the body needs to soften the endometrium (mucus lining of the uterus). This makes it easier for the egg to attach if fertilized, or releases the endometrium if conception doesn't occur (the uterus then sheds the endometrium) and your period starts.
An increase of progesterone levels in the body also increases the temperature. If the egg is not fertilized, the corpus luteum stops producing progesterone after approx. 12-16 days and your temperature drop to below your cover line again.
The luteal phase is between 10-16 days, the average length is 14 days and is usually equally as long in every cycle. If you have a luteal phase defect and your luteal phase is less than 9 days, it may be harder to get pregnant. You can read more about this here.