It means to have a drink and send it away. Don't dwell on the past, and it won't end if you start over in the future.

Let's drink to the past. The past does not look back, and the future does not end. May you have no waves in this life and respect me for the rest of your life.

It is mostly used to express the vicissitudes of people who have experienced many things and intend to bid farewell to the past and start over.