A vocal solo occurs when an artist decides to vocally improvise during a song. They are marked by either intense and sudden crescendos or quick octave fluctuations.
Vocal solos are very difficult to perform because creating a new set of lyrics on the spot is something many artists cannot do. As a result, vocal solos often feature artists singing indiscernible lyrics or repeating a word/phrase.
Also see: "Bonus chorus
"I thought the shat had hit the roof
until Aretha laid down that vocal solo... Improvising was a good move because that shit was bunk. But it was powerful, man! It brought a tear to my eye."