Lester has a cavity and they "remove" his tooth. They have a large tooth model with cavity in it to explain why a tooth is painful if it has a cavity, why a cavity is bad, and what a dentist does to repair the cavity.
This representation of cavities and bacteria shows workers chipping away at King Dedede's teeth. Dedede's reaction shows how painful a cavity can be: it affects his sleep. Dedede and Tuff are both scared of going to the dentist and try to avoid it.
King Dedede brags to Escargoon about eating all the junk food he wants without getting cavities. Back at the house, Tuff has a similar mindset as King Dedede, stating that it's not important to brush his teeth. His family, and Kirby, show him the right way to brush his teeth. Despite their example, he doesn't brush his teeth properly.