It means that there's no point caring too much and being sad about what happened in the past because it cannot be undone or changed.

Once something bad happened and you cannot fix it, it's better to stop whining and move on... you won't make things change by 'crying'.

Some situations cannot be changed, no matter how much you might want to change them.

What has passed has already passed. It can not be returned.

As the others have said, it means there is no point in being emotionally attached to something you cannot change. The phrase refers to a situation in which you spill milk, once spilt, you cannot 'pick it up' and use it again, and no amount of effort (or crying!) will change that.