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.