Proper adjustment of your banjo's truss rod allows you to put a little bit of concave curve in the neck of your banjo to make the playability a lot easier. This is called "relief".
The relief affects the "action" or string height of the strings from the fingerboard. Lower action can be easier to play, but if your action is too low, you can get buzzing from the strings hitting the frets. Higher action can allow you to play with a harder attack and produce more volume, but it will be harder to fret the strings.
Deering Banjos Quality Control Manager Chad Kopotic walks you through how to adjust your truss rod on your banjo in order to have the proper relief in the neck of your banjo.