Filch is the cantankerous caretaker of Hogwarts School of Witchcraft and Wizardry. He is the creator of the best cat name of all time. He also happens to be a Squib ― a non-magical individual who is born to a magical parent.
As caretaker, Filch is tasked with overseeing the cleanliness and general upkeep of Hogwarts. This job involves patrolling the corridors at night, giving students detention and maintaining the overall hygiene of the school. And as a Squib, Filch has to do that job ... without magic.
Let's go over that again. Filch has to keep an entire freaking castle clean without using magic to do it.