Leave a comment right on a block, or on any spot of the canvas: press C (or Pin to a block in the Comments panel), click where it belongs, and write. Type @ to mention a teammate.
Everyone who can open the flow sees a small pin there, with how many messages it holds, and it appears on their screen as soon as you post. Click it to read the conversation and reply.
Resolve takes a conversation off the canvas. The Comments panel keeps every one, open and resolved, and says on which block each was left.