The Lethargarians live in the Doldrums, and they appear as a warning to Milo about what can happen if you're too bored or inactive. Sounds like just the kind of lesson Milo needs to learn, huh?
At first, it might seem like a lot of fun to have no responsibilities and to just float along with no chores or tasks to accomplish. But, these people are so obsessed with being lethargic that they even have a schedule for their nothingness activities. No joke. They plan out their days to make sure they get nothing accomplished. There's "'almost no time for brooding, lagging, plodding, or procrastinating, and if we stopped to think or laugh, we'd never get nothing done,'" they inform Milo (2.65). Their whole purpose in life is "to get nothing done" (2.67).
It turns out that by doing this, the Lethargarians have lost their color and sense of self. They finish each other's sentences and almost seem to have a group mind. They don't have any individuality, and the longer you stay around them, the fewer thoughts you have. The key to getting away from them, Tock shows Milo, is to think as many thoughts as you can. Thoughts banish the Lethargarians and get you out of the Doldrums. Keep that in mind next time you're feeling like you're down in the Doldrums.