What does the serving of tea mean socially?

From the text:

"'If you want to survive in Baltistan, you must respect our ways,' Haji Ali said, blowing on his bowl. 'The first time you share tea with a Balti, you are a stranger. The second time you take tea, you are an honored guest. The third time you share a cup of tea, you become family, and for our family, we are prepared to do anything, even die,' he said, laying his hand warmly on Mortenson's own. 'Doctor Greg, you must make time to share three cups of tea. We may be uneducated. But we are not stupid. We have lived and survived here for a long time.'