A singer may use his chest register rather comfortably in his lower range, but feels tight and restricted if he tries to use that placement, or that set of muscles, to try to sing higher. That is his BREAK or PASSAGGIO. Some singers may have figured out a lighter higher placement (using different muscles) to sing in their high range. The sound may be beautiful and well centered, but as they descend, they lose all control because they have not learned to "shift gears" to allow different muscles to act to sing the notes in that middle register, or bridge the gap between their high and low voice qualities. They might even yodel between the two registers in a very uncomfortably embarrassing way. These singers may, in fact, have two completely different sounds in their head voice and chest register with no control at all in the mid-range.
A good voice teacher will work tirelessly to help the student work through navigating the PASSAGIO until the entire range is unified. And then, when the singing gets tough, or when the body is tired, the singer can fall back on good technique to see him through. When singers abuse their voices and do not navigate well the PASSAGGIO, bad things can happen. The worst is developing vocal nodes or scar tissue build-up on the vocal folds. You don't want that! So, if you suspect that you may be experiencing problems NAVIGATING the PASSAGGIO and are at your BREAK-ing point, RUN, not not walk, to a good vocal teacher. She will see you through!