What does "Visit Waypoint" in missions mean?

A waypoint is a designated location, usually one of several that you need to visit in order to complete the mission. Field Trip locations require you to open the "card" that links to the Field Trip page in order to complete.

Depending on the particular mission, the order in which you visit each waypoint might also be important, so you may find that they don't complete when visited out of the listed order (these ones are marked "complete waypoints in sequence" on the Details screen). Sometimes the order is hidden and linked by clues (and some require actions to be performed, such as using the "Navigate" function).

Source: Google - Experience Ingress Missions

Source: The Verge - Google's 'Ingress' will soon let players build scavenger-hunt missions across the globe