The Hall is a vestige of the main room in castles and other great medieval buildings (such as Westminster Hall). All traditional Oxford and Cambridge Colleges have them – the colleges have retained the tradition of everyone eating together, as have the Inns of Court and many public (ie private) schools. Such halls are often now called Great Halls or Dining Halls since the idea of a hall has changed to no more than an entrance or vestibule. (“Refectories” are for monasteries and aspirational modern institutions)
The tables here have individual seats rather than the traditional benches. Until recently, before Health and Safety moved in, it was normal practice for undergraduates to walk over the tables to get to their benches on the other side of the table. You can just see the High Table at the far end.
Christ Church College is usually called just Christ Church, or for those in the know “The House”. It was originally called Cardinal’s College after its founder Cardinal Wolsey. It still bears his arms. It has a certain prestige – it was re-founded by Henry VIII, it has produced 13 British Prime Ministers and its chapel is a cathedral.
The hall here was used as a model for the hall at Hogwarts in the Harry Potter films. The college is the setting for parts of Evelyn Waugh’s Brideshead Revisited, as well as part of Lewis Carroll’s Alice’s Adventures in Wonderland, and also the film adaptation of Philip Pullman’s novel Northern Lights.