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This is basically an idiom that is rarely used today, but it actually means: You (or someone else) have/has made a big mistake, that this mistake is almost impossible to fix or will take a long time to; and has inevitably created a pandora's box; and/or has opened up a can of worms.

In other words, it is basically like making a huge mistake that will take almost forever to clean up. (Spilled Milk is the metaphor for the mistake.) Similar to: drop the ball
(Ex. It's easier to spill a glass of milk, but not so easy to put back all of the milk into the glass. It's easier to create a mistake, not so easy to clean up the mess of the mistake overnight. etc)

In the 1960s, civil rights activists accidentally spilled the milk on the Black (African American) community. Now, many blacks and latino's in America are in poverty and are divided into rivalry street gangs to keep each other down.
by Helba the Hacker December 09, 2018
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