Filth is dirt, gunk, goo, dust, hair, poop, cobwebs, dead bugs, crumbs of food, or any other stuff that's totally gross and needs to be cleaned up. And if something is covered in filth, we call it filthy. In other words, filthy things are gross, nasty, and need to be scrubbed clean.
If I didn't clean inside my fridge for two years, it would get filthy! There would be smears of dried orange juice on the shelves, and blobs of crusty hummus in the drawers. I'd open the fridge one day and think, "EW. I've really got to clean off all this filth."
Have you ever visited a filthy public restroom? There's mud and dirt on the floor, hair and chunks of food in the sink, and pee and poop everywhere. GROSS! Back away from that filth! No matter how badly you need to use the bathroom, don't use a filthy one. Ughgh.
Filth can also be books, movies, or other materials that are nasty or gross, or not appropriate for kids. In Home Alone, with no adults around to stop him, Kevin watches a filthy movie, with lots of violence and cursing.
"Filth" and "filthy" are extremely strong words. If you say that animals or people are filthy, you're saying they're nasty and disgusting: a very strong thing to say. If I say that Miley Cyrus gave a filthy performance, I'm saying her dancing and her body language were gross and inappropriate—because so many of her fans watching are kids.