Other forms: crying over spilled milk; cried over spilled milk; cries over spilled milk
When you cry over spilled milk, you waste time regretting something you can't fix or change. Milk can't be unspilled, so you might as well just get on with the cleaning up.
The phrase has a less common variant form: cry over spilled water. Both are usually used in the negative: Don't cry over spilled milk. It's similar to water under the bridge, used for something that's already happened and usually has already been forgiven. Spilled milk may or may not be forgiven, but there's nothing that can be done to fix it, so instead you have to work on changing what you can.