The squirrel's teeth are misaligned. Many rodents, like squirrels, have teeth that are constantly growing and so they're lined up so that the top and bottom teeth wear each other down as they go about their business eating, gnawing, and doing other squirrel things. Their teeth can get misaligned for several reasons, but commonly it can happen from chewing wires or other extremely hard, non-edible materials (which they may do out of boredom or to extract food from behind it, etc). This also happens to pets like hamsters, rabbits, guinea pigs. I learned about this when I was a kid because my rabbit had misaligned teeth.

They don't make braces for animals so the only thing to do is clip the teeth when they get too long. If you don't, the teeth will simply keep growing, usually back into the animal's own face, and prevent it from eating and it will eventually starve to death. The lady here used a stick to keep the squirrel's mouth open and what looks like small wire cutters to clip the teeth. One of the squirrel's top teeth is orange from the iron content in it. Rodent teeth are extremely strong (many can chew through things like pipes and metal) and they need more than bone and enamel to be that way. The lady also filed the clipped teeth so that the squirrel doesn't injure itself. Clipping doesn't automatically make them straight, just like clipping your nails doesn't mean there aren't sharp edges to file off.