1. forgive or agree to forget about a past conflict or disagreement with somebody
it's about time we let bygones be bygones
This expression is akin to a peace offering. You are requesting or imploring somebody who you have had previous conflict with to wipe the slate clean. To let whatever happened in the past stay there and no longer impact the present or future. This can be the case whether there is blame to be shared, or whether one party is totally at fault. It's commonly used in a social context with regards to forgiveness or reconciliation between friends or family. In a professional environment, it is more likely to be used for financial or other practical reasons.