Other forms: going with the flow; went with the flow; goes with the flow; gone with the flow
If someone arranges a get-together without any planned activities, saying, "We'll just go with the flow," they mean that the group will be flexible and do whatever most of them feel like doing at the time.
Going with the flow is like floating on the current of a river: You go where the current takes you. It might mean seeing what everybody else in a situation is doing, and copying them. If you've never been to a fancy banquet before, and aren't sure which fork to use when, or what to do when somebody proposes a toast, you might just go with the flow, watching what everybody else is doing and doing that, too.