"Vratham" means "vow". The Sathya Narayana Vratham is a ritual performed for wealth, education, prosperity, offspring, success, etc. It is often performed on Ekadashi, as it is here, with 1108 couples taking part. As the priest mentioned in his Telugu speech, the ritual was being conducted for the well-being of all the beings in the worlds (Samasta loka sukhino bhavantu).
The legend is that Sage Naradha was distressed at the misery of men in this world, and Lord Vishnu said that this ritual, performed to Lord Sathyanarayana, would relieve people of their troubles.
Bhagawan enters Sai Kulwant Hall almost at the conclusion of the ritual and sits through Arathi and prasadam distribution.
For more details, see sssbpt.org/Pages/Prasanthi_Nilayam/alwarfestivalreport.html.