Mission starters are NPCs that allow players start missions by talking to them. They are found in mission outposts in Nightfall and Eye of the North. They can be identified by a green shield icon floating above their heads. When you talk to them they will ask if you are ready to begin the specific mission they are offering, which you may answer in the affirmative or negative to. If you or any of your party members doesn't meet the requirements to start the mission they will tell you what is needed.