It's a natural slope in the ground. A large screen is placed at the top so the slope permits a sizeable number to watch the matches.
If you are wondering why the people would want to watch a screen inside Wimbledon. It's because the better matches take place on centre and number one courts but most of the spectators only have admission to the many lesser courts.
The majority of centre and number one court tickets go to members, debenture holders and the marquee packages.
I was very fortunate in being able to go with my wife for 14 years because the company I worked for had a marquee there.