Guinea pigs are rodents, and like other members of that family they have long teeth to break up their tough, fibrous food. The front teeth grow constantly throughout the GP's life, and if the animal's diet lacks tougher materials, this can cause problems of overgrown teeth.
In extreme cases the guinea pigs will need their teeth filed to enable them to eat properly. This isn't a big operation, usually, and a vet can carry it out as a routine procedure. The giveaway sign that a guinea pig needs its teeth filing is when it seems interested in food but is unable to eat, or has trouble doing so. Obviously, this is a problem that needs dealing with quickly.
A Guinea Pig's teeth grow throughout the animal's life
Guinea pigs can be given hard chews that enable them to keep their teeth at the right length. Be careful if supplying a wooden chew yourself, as some types of wood are harmful to guinea pigs. Do your research first. Oak, cherry, apple, and poplar are all common woods that are safe for guinea pigs. Cedar and pine are amongst the woods you should NOT give as a chew, as they are toxic for guinea pigs.