In singing, back space refers to the space in the back of your mouth and in your throat. Just opening your teeth or the front of your mouth (front space) shows off your gorgeous pearly whites, but it doesn’t do enough for your tone.
Yes, you do have to open your teeth to create enough back space, but the big opening has to be in the back of your mouth and your throat. For space and breath to work together, you need to open the space quickly and then move the breath.
To create the open space necessary for great tone, pretend that you have an egg in the back of your mouth. You can use other images, such as a golf ball, if you don’t like eggs. Compare the feeling of the space when it’s closed and when you have the egg sitting on the back of your tongue. Practice opening that space quickly. Allow your tongue to release down, not press down.
Try singing part of your favorite song. Find the openness from the imaginary egg in the back of your mouth, and begin singing with the throat and the back of your mouth open. Remember to find the same clarity that you had when you were humming.
You can compare the tone change by first singing with the throat and back of the mouth closed and then singing with the space open. If you aren’t sure about the difference in the sound, record yourself and listen.
The first few times you open the space in the back of your mouth, the resonance may sound hooty, as if the sound is in the back of your mouth. Allow the tone to be hooty while you learn to coordinate the space. Eventually, you will be able to open the space and send the sound forward.