Meaning that it is of no use to dwell on what should have (shoulda), could have (coulda) or would have (woulda) happened/been done.
Said as an attempt to shorten a discussion that focusses on the past, thus providing no solution to an actual problem.
"Man, if only I didn't cum straight in the air, I wouldnt have jizz on my mullet now."
"Shoulda coulda woulda. There are more important things than that. There is more to life than the jizz on your mullet. You're still young, someday you will wash your hair and the jizz will be gone, but your mullet will last forever."