The time of your birth when the moon is transiting through the zodiac sign in cosmos and a nakshatra inside the comos , then its called Janam nakshatra.
The Birth nakshatra shows the mind of the native. Mind is the controller of the actions. This nakshatra also controls the longevity (life) of the native . It has so profound effect on natives thinking that all his actions are surrounding the qualities of that nakshatra.
Below is the nakshatra Table which shows the aim of the nakshatra native (dharma, artha , kama , moksha) and also the power associated with that nakshatra.
Each of the 9 graja owns 3 stars which makes a 27 list. e.g in Below Table , if you are born in Uttarphalguni then the same will have a wealth effect due to Sun being lord of this nakshatra.