There are 9 main physical openings in the body which include 2 eyes, 2 ears, 2 nostrils, 1 mouth, 1 anus and 1 urinal opening. The Dasam Duaar (Tenth Door), is the tenth opening of our body, but it is not a physical opening. Rather, it is a metaphysical or spiritual one. The purpose of Dasam Duaar is to connect to God and to experience God. It is a way for us to truly experience God’s existence. The Dasam Duaar is hidden and we can only open it and experience God if we read Bani, connect to Naam and reach out to God with love and devotion. 

One example of Guru Ji describing the Dasam Duaar is in Anand Sahib when Guru Ji says,

ਵਜਾਇਆ ਵਾਜਾ ਪਉਣ ਨਉ ਦੁਆਰੇ ਪਰਗਟੁ ਕੀਏ ਦਸਵਾ ਗੁਪਤੁ ਰਖਾਇਆ ॥

Vaheguru blew the breath of life into the physical body, and revealed the nine doors; but They kept the Tenth Door hidden.

ਗੁਰਦੁਆਰੈ ਲਾਇ ਭਾਵਨੀ ਇਕਨਾ ਦਸਵਾ ਦੁਆਰੁ ਦਿਖਾਇਆ ॥

When one comes to the Guru’s Door with loving devotion, then the Tenth Door is revealed to them. 

You can watch Bhai Jargaj Singh discuss this in detail in the video: