The roof is on top of the fictional apartment building at 2311 No. Los Robles Avenue where Sheldon, Leonard and Penny reside and has been used as a location for various "The Big Bang Theory" episodes.
Significant appearances in episodes
In "The Nerdvana Annihilation", the guys are bringing their new time machine upstairs, blocking Penny's ability to go downstairs on her way to work. She ends up going up to the roof, crossing the gap between the building and the one next door, entering another apartment and having lunch with an Armenian family. She missed work and also got fixed up with their son.
In "The Lunar Excitation", the guys are bouncing a laser off the moon using a laser set up on the apartment building roof. Penny joins them and brings her date, the dim witted Zack Johnson. Sheldon is forced to go on a date with Amy Farrah Fowler because Raj claims that he held a dirty sock from the roof in his apartment.
In "The Countdown Reflection", Howard and Bernadette have a quick wedding on the roof so that Howard can make his trip into space and so that the Google satellite will pass overhead and photograph their wedding. The roof is decorated with a heart and arrow pattern and is seen in full as the view pulls back from the building to the neighborhood and finally through California and up to the whole Earth.
In "The Novelization Correlation", Sheldon ran up to the roof to shout out his frustration after Amy appeared on his favorite television show.