What does "Water under the bridge" mean?

What does "Water under the bridge" mean?

It means that something is in the past and no longer important.

"My sister and I fought when we were children, but that's water under the bridge."

I think the analogy is that water under the bridge is constantly moving toward the sea. That's a little abstract, so you might also imagine dropping a leaf into the water from atop a bridge. By watching the leaf float down river you'd be witnessing the progress of the water.