At each hole on a golf courses, there are multiple tee boxes, usually designated with colored tee markers. Through much of the 20th-century history of golf, it was traditional at many courses to use three tee boxes, designated by the colors red, white and blue. Red tees were the forward tees, white tees the middle tees, and blue tees the back tees.
Today, many courses use four, five, six or more tee boxes per hole, and the traditional colors are not necessarily used at all; or, if they are, don't necessarily correspond to their traditional placements. A blue set of tees today might be anywhere on the teeing ground, from forward to middle or back.
But the traditional meaning of "blue tees" is still something that is used as a synonym for the championship tees. If you read a generic reference to blue tees, or hear the term in conversation, the reference is probably to a golf course's back set of tees.