Agile is a collective term for various software development methods. Instead of thinking out a project from A to Z and only then starting it, agile software development opts for dividing a project into short periods of a few weeks. New results are always delivered in these sprints. Collaboration, continuous planning and continuous learning are the main points of attention. It's at odds with methods that try to deliver everything at once at the end.
Agile focuses on keeping the development process lean and creating minimally viable products that go through a number of sprints before something is final. Feedback is continuously collected and implemented and all in all it makes for a much more dynamic process where everyone works together towards one goal.