Music is based on two notions that can be found in all musical styles, which are: tension and release.
Music is a clever mix between these two notions.
Indeed a song with only tension would be stressful and unpleasant while a song with only release would be boring.
We must therefore play with these two concepts in order to create relief, harmonic rhythm.
In this tutorial you will find a small example which nicely illustrates the tension and the release.
At the beginning of the phrase on the tongue drum I play the D (which is the fifth of G) then I alternate between the B and the A (which are the third and the second of G).
This whole part of the phrase represents tension, and the release comes when the G (the tonic) is played on the tongue drum.
You will find a little further down the score corresponding to this tutorial, you just have to click on it and "save the image as" to download the score.