Influence without authority
You must be influential without having authority. This is something that professional PMs work at for a lifetime.
Clear and influential communication is possibly the biggest necessary skill and the hardest challenge that a PM faces, because you are directly responsible for the success of your product, but rarely directly in charge of the people who are doing the bulk of the work. I.e., You are a leader to the team, but not their boss, which forces you to meet a higher bar of convincing people that your strategy/features/documents are really correct.
You have to be the person who leads from the front and "brings the donuts" - you are the anchor, spiritual leader of the team and your job is to make sure everybody is happy, healthy, productive, knows what they're working on, and why they are working on it.