Sthana is a technical term in Hatha Yoga. It means the brahma-randhra, which is the opening at the topmost portion of the skull. This explanation of the word sthana is mentioned by Brahmananda in the commentary of Hatha Yoga Pradipika (II.12). It is said that the highest and best measure of kumbhaka (holding of breath inside), which means each kumbhaka of forty or more seconds when practiced four times a day with eighty kumbhakas in each sitting, enables the prana (vital air) to reach the brahma randhra (uttame sthanamapnoti) (II.12). This result may be achieved within a period of a few months or years, depending upon how far one succeeds in making the nadis cleansed by pranayama.
This process is explained further in the fourth chapter of Hatha Yoga Pradipika (IV.76). It is said that when the prana (vital air) opens up the knot situated between the eyebrows, called rudra granthi, the highest state of silencing the mind and prana comes into being. That state is called nishpatti avastha. Hereafter, the prana reaches the brahma-randhra. That is said to be the culmination of the practice of raja yoga (IV.77). That is achieved through four steps.
The first step is the arousal of the kundalini power. The second step is the arousal of kevala kumbhaka. The third step is the arousal of nada and the fourth step is the granthi bheda (opening up of the knots). Then the sthana is reached when prana reaches the brahma-randhra. This is also explained in great detail in Satcakrani rupana.