If you've follow all the videos up to this point - then you've already met the aeolian mode. It is just another name for the natural minor scale.
You can build an aeolian scale by playing all the white notes on the piano between A and A.
Lots of composers have used the aeolian mode to create both melodies and harmonies.
At the end of the video I improvise a piece based on the aeolian mode and also make use of a tierce de picardie. This is a technique we can use to make a piece in a minor key more interesting. We do this by finishing the music on a major chord rather than and minor chord, which we would be expecting.