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.