For those who don't watch, Sheldon is the tall, skinny, nerdy guy who is smarter than everyone on the planet and knows it. He has a superiority complex and is a germophobe. Did I mention nerdy? Star Trek, Star Wars, comic books, computer games, etc.
Leonard (aka David from "Rosanne") is Sheldon's room mate. He is also very smart but a little more street smart than Sheldon. Sheldon and Leonard work together in the science laboratory at the local university.
Penny is the girl who lives across the hall from Leonard and Sheldon in the apartment complex. She is an aspiring actress and is very much NOT a rocket scientist. She works at a Applebees-ish restaurant. She and Leonard dated briefly.
In this episode Penny has made it known that she has bought Christmas gifts for Sheldon and Leonard. Sheldon has a fit because of the implied reciprocity and has dragged another two of his pals (Howard and Rajesh) to a foofy store to buy a gift for Penny. Not knowing what type of gift Penny is getting for him he is unsure how much he should spend on the gift.
He devises a plan that should allow him exchange a gift appropriate and commensurate with the gift he will receive from Penny -- he will purchase several gifts of varying value. Once he receives his gift he'll feign intestinal difficulties... when he returns from his business he'll bring back the appropriately valued gift for Penny.