Speech bubbles in comics or graphic novels are essential tools that are used to convey dialogue that is being uttered aloud. They are one of the most important elements because they reveal what an author is going to depict in a story and how characters vary from one another.
The bubble is also depicted in different shapes indicating someone may be yelling (pointy), thinking (cloud-shaped), whispering (dotted lines), or speaking in a manner different than the other characters (different font, bubble shape).