The distance between any two musical notes is called and interval. In Western musical notation, the smallest interval is the half step, or semitone. A whole step, by extension, is an interval equal to two half steps.
Musicians combine half steps and whole steps in specific patterns to create specific scales, the concepts of half steps and whole steps are the most basic principles of music you must understand in order to create a pleasing melody or harmony.
Half steps
Half steps are more straightforward on the guitar than many other instruments. Each fret is a half step. You just move one fret up or one fret down from your starting point, and that move of one fret equals one half step.
Moving down the neck (toward the headstock of the guitar) flats the note, while up the neck (toward the body) sharps it.
Whole steps
Following the logic that a half step on the guitar is one fret away from the starting point, it only makes sense that a whole step would be two frets away from the starting point. So, on the guitar, playing whole steps is represented by a move of two frets up or down the neck.
Major and minor scales contain more than twice as many whole steps as half steps, but they are both important to how the scale sounds, so be sure you're familiar with both the half step (or semitone) and the whole step.