The word litter has several meanings:

1. Trash carelessly scattered in public places instead of being thrown away properly. When someone drops a candy wrapper on the sidewalk or tosses a soda can in the park, that's litter. It makes neighborhoods look messy and can harm animals who might eat it or get tangled in it. Most communities have laws against littering because everyone shares responsibility for keeping public spaces clean.
2. A group of baby animals born at the same time to the same mother. A cat might have a litter of five kittens, or a dog might have a litter of eight puppies. Scientists and breeders talk about littermates when they mean animals from the same litter. When you hear that someone's puppy came from “a litter of six,” it means the mother dog had six puppies all at once.
3. Material spread in a box for indoor pets to use as a bathroom. Cat owners put cat litter, usually made of clay or other absorbent material, in a litter box so their cats have a designated indoor bathroom spot that can be easily cleaned.

As a verb, to litter means to leave trash in a place where it doesn't belong, like littering a beach with plastic bottles.