Tonality

Tonality is the grouping of pitches and chords into a hierarchy of relationships. 

In each tonal group or key, there is one tone which sounds the most stable, like a 'home' pitch. This pitch is called the tonic, and the name of that tone is also the name of the key.

For example, in the key of C Major, the tonic pitch is C. When you play along with a musical piece that is in the key of C Major, you can play the tone C at just about any point in the piece and it will sound like it 'fits' without much dissonance. 

Each key has a scale of tones associated with it. The tones that make up a scale are typically ordered by the frequency of their pitch. A scale that moves from lower pitch tones to higher ones is called an ascending scale, while a scale that moves from high to low is called a descending scale.