Complex sentence: "It does exactly what it says on the tin" is a popular phrase in the United Kingdom, which originated in a series of television advertisements by the woodstain and wood-dye manufacturer Ronseal, initiated in 1994 and still being broadcast.
Split into simpler sentences: It does exactly what it says on the tin was originally an advertising slogan in the United Kingdom, that then became a common idiomatic phrase. It originated in a series of television advertisements by the woodstain and wood-dye manufacturer Ronseal, initiated in 1994 and still being broadcast.