When you say that something is ‘water under the bridge’, you are saying that something is in the past and is no longer worth referring to or relevant. The image is of water moving on and not staying stuck in the same place. When something is ‘water under the bridge’ you mean that the problem that occurred in the past has been dealt with and is not worth dwelling on now. This is a popular idiom used in both social and professional contexts.

1. Don't worry about the argument we had last year. It's all water under the bridge now as far as I'm concerned.
2. There was a time when I really didn't like Steven, but that's all water under the bridge.

1. That supplier did let us down with a shipment a while back, but they've since apologized and reimbursed us so it's water under the bridge.