A scale is a series of closely spaced notes in ascending or descending order.
The most common scale is called the C Major Scale.
The Major Scale acts as a ruler. On a normal ruler the numbers are evenly spaced.
In a Major Scale, however, the numbers are not evenly spaced.
A Half step is the same size as a Minor 2nd, or a Semitone.
A Whole step is the same size as a Major 2nd, and is equal to two Half Steps.
Whole Whole Half, Whole Whole Whole Half.
Any time you play the white keys in ascending order starting on C, you have a C major scale.
On the guitar you can think of each key you see here as 1 fret.