Like all rodents, and like rabbits too, guinea pigs have teeth that keep on growing. They are usually worn down by gnawing and chewing on fibrous foods.
If you look at your guinea pigs' front teeth (two at the top and two at the bottom), you may be surprised by how long they are. Each tooth should be between a third of an inch and just over half an inch long, and they should meet evenly. Given this set-up, the teeth will wear down evenly.
Guinea pigs' teeth keep on growing
Sometimes, however a tooth may break, as the result of an accident, or from chewing on the hutch wires. If this happens, the other teeth may need to be trimmed to make them all even again and enable the GP to eat easily.
As the guinea pig gets older, its back teeth may become overgrown. This is not very common, but you need to be aware of it as a potential problem. Any guinea pig that stops eating should be taken to see a vet as soon as you notice the issue.