Please answer with a simple explanation and example.
Why is every 5th tone from tonic called "dominant"?
The dominant degree is called dominant because it is so important. Why is it important? It's important because it 'wants' to go to the tonic. In western music we have got used to dominant to tonic sounding 'finished' or at rest. It is this 'pull' to go from the dominant to the tonic that underpins melody and harmony, and that's what makes the dominant so, well, 'dominant'.
A chord built on the dominant (a V7) badly wants to go to the tonic chord (I) because V wants to go to I. What's more, the other notes in a dominant chord want to go to the notes in a I chord. So, a G7 chord really wants to go to a C chord. This V-I pull 'dominates' musical theory and functional harmony.