The NavfnPlanner is a global planner plugin for the Nav2 Planner server. It implements the Navigation Function planner with either A* or Dij. expansions. It is largely equivalent to its counterpart in ROS 1 Navigation. The Navfn planner assumes a circular robot (or a robot that can be approximated as circular for the purposes of global path planning) and operates on a weighted costmap.
The global_planner package from ROS (1) is a refactor on NavFn to make it more easily understandable, but it lacks in run-time performance and introduces suboptimal behaviors. As NavFn has been extremely stable for about 10 years at the time of porting, the maintainers felt no compelling reason to port over another, largely equivalent (but poorer functioning) planner.
See its Configuration Guide Page for additional parameter descriptions.