This is a
phrase that is generally used in an
argument or when making a point. Basically it means that it doesn't matter what happened in the
past, because the past is the past. What is important is what happens
now.
It is also the title of a novel by S.E. Hinton, who wrote The Outsiders. The book was made into a movie in 1985 that starred Emilio Estevez and Morgan Freeman.