One of the most popular articles on this site is Chris’s Very, Very, Very Basic Understanding of Music for Absolute Beginners.
The other day he spotted an excellent YouTube covering the same topic from film, games & TV composer Guy Michelmore, who explains it whilst playing examples on a keyboard:
Join composer Guy Michelmore as he explains the essentials of music theory in just 16 minutes! What is an octave? How do you make scales? What are intervals? What’s the difference between major and minor? What is the circle of fifths?! All explained in less time that it takes to make and drink a fresh cup of tea.
A free supporting guide Music Theory In Under 30 Minutes can be downloaded from his main website too.
Guy has many interesting & informative videos on his YouTube channel including a playlist of Music Theory ones; how to write music and working with sound samples.
Here is Music Theory in 16 Minutes :