Bell-bottom jeans are a type of jeans where the jeans flare out from the knees in a distinctive bell shape. Originally worn by sailors in the navy, bell-bottoms became popular in Europe and North America during the 1960s and 1970s.
Bell-bottoms were originally a high-fashion statement, but were quickly adopted by the hippie counter-culture movement, with which they are now more commonly associated. They were also popular during the disco culture of the ’60s and ’70s, so they have that connotation as well.
Bell-bottoms have fallen out of popularity in modern times, especially among men, and should be avoided unless you are going for a retro/disco look or a hippie/throwback look.