Basic music theory is important to learn if you want to understand how music works. Learning this information can help you play, compose, and appreciate all forms of music. Some musicians learn to read notes on a musical scores, while others learn by listening to their favorite songs and figuring out the notes on their respective instrument by ear. Both methods can benefit from learning the theory of the scales and chords that are woven into all the music we enjoy.
Major scales - Knowing major scales is like knowing your multiplication tables in math. They take a while to learn (though there are only 12) and apply in one way or another to many different aspects of basic music theory from building chords to intervals and eventually chord progression analysis. Here is an outline of how they're structured which is a simply a series of half and whole steps.
Minor scales - These scales come from a similar structure as major scales but start on a different note, and result in a much darker or even sadder quality. Here are the three common types of minor scales and their various uses.
Music intervals - This is the term to describe the distance between 2 notes. Each distance has it's own name which is derived from the major scale. It is beneficial to musicians and composers to be able to identify intervals in notated music and/or by just hearing them.
Triads - These are the basic 3 note chords which are the building blocks of much popular and classical music.
Seventh chords - These are fuller 4-note chords as they contain the seventh note of the scale they're derived from. There are a few types and are very common in jazz and other genres.
Suspended chords - A suspension is when the third note of the scale is replaced with the fourth note (or sometimes second note). They can either be used in a classical sense to eventually resolve to their respective major chord or in more contemporary music (post 60s) as well as impressionistic music, they provide an interesting ambiguous type of chord that can be used in a variety of artistic fashions.
Chord inversions for voice leading - Inverting chords means playing the notes in different orders instead of just having the root be the bottom note. This is useful when voice leading which is how you get from one chord to the next without necessarily jumping around. Playing chord progressions with good voicing lead results in a more developed, sophisticated sound.
Circle of Fifths -Understanding the circle of fifths is a useful way to learn how all 12 different keys relate to each other as well as being a useful way to learn key signatures.