IKEA is a Swedish company specialising in the production and sale of furniture. Their use of 'flat-pack' furniture, which requires the consumer to build the furniture themselves, and large out-of-town warehouse-styled stores have made them a cheap and popular place to buy furniture across the world.
Relevance to the show
In 0102, the flat-pack entertainment center which caused Sheldon and Leonard so much trouble getting up the stairs was, according to Penny, a "cheap Swedish media centre", which almost certainly means that it came from IKEA (Bill Prady referred to the media center as being from IKEA in an interview, actually).
Just for the record, IKEA delivery people do bring the boxes up to your flat, even if it's on the fourth floor.
The closest IKEA store to Pasadena is at 600 N San Fernando Blvd, in Burbank.