In Common words 2S Same as 2X2 seating in Buses. Most of the Intercity trains will be having 2S class.
Above image shows 2S seating arrangement. You Can Reserve 2S same as Sleeper Seat and Charges are 60% Less compared to Sleeper Class for the same Journey/Type Train.
This coach does not provide the facility of sleeping, there are no berths. Passengers need to travel on the chairs. Thus, quite okay for 4-5 hours journey however, passengers do travel for overnight journeys too.
If you Have Any Question Regarding Second Seating, Please Ask through Comment.