Origin
This expression could be found in writing in a book called Banking Under Difficulties in 1888.
Explanation
It means there’s no use in worrying over past events which cannot be changed. It is said to emphasize that it is not useful feeling sorry about something that has already happened. There is no point in regretting something that has already happened and cannot be changed or reversed. You should not get upset or feel sorry about something that has happened which you cannot change or fix. You should keep moving forward instead of looking back at something bad which happened.“Spilt” is the past participle of the verb “to spill”, which means to accidentally drop a liquid, so “spilt milk” (or “spilled milk”, which is more common in American English) is milk that you have dropped and now cannot get back. There is no use crying over it because it won’t change the situation; the problem is a small one, and it is therefore better to move forward positively than to waste time worrying about it.
Examples
I know you didn’t mean to break my phone, so it is no use crying over spilt milk now.
Sometimes I regret not finishing my degree. But it is no point crying over spilt milk.
My kids were upset because they had burnt their toast, but I told them that to just make some more. It’s no use crying over spilt milk.