Blue key generation is a process where a subject is filmed or photographed against a background consisting of a single colour - Blue. The portions of the video which match the blue background are replaced by the alternate background video. This process is commonly known as "keying", "keying out" or simply a "key".
The Blue Key process is often referred to simply as 'Bluescreen'. Blue is often used as it is the complementary colour to flesh tone - since the most common colour in most scenes is flesh tone, the opposite colour is the logical choice to avoid conflicts.