Lint is a collection of dust particles and tiny, loose bits of the materials your clothes are made of (throw in so human and pet hair for good measure).
Dryers tumble your clothes while blowing hot air into the tumbler drum. This dries your clothes but also loosens the dust and lint from your clothes, floating into the air.
Meanwhile, the dryer is extracting that same sir out through a vent. It passes through the tight mesh lint filter before heading outside. Through your vent. Most of the lint particles are larger than the holes in the filter so they become trapped, until you clean it.