Lint is made of clothes fibers that come off during the washing and drying process. The lint trap or filter collects this lint with the aim of preventing it from getting into the vent.
The location of your lint trap will vary depending on the dryer manufacturer, but they’re usually in an accessible spot, like just inside the door or on top of the dryer. Check your manual for specific location and cleaning instructions for your model.
The lint trap is a mesh filter screen that usually pulls easily out of a slot in the dryer. Any lint collected on the screen can be removed by hand. Best practice is to do this after every load. But for a more thorough clean, you can gently vacuum it using a brush attachment on your vacuum.
Before putting the screen back into the slot, periodically check if any lint has accumulated inside, and if necessary use a long attachment on your vacuum to remove it.