The lint that you are seeing is coming from your dryer vent and is likely escaping while a load of laundry is drying. While one purpose of the dryer vent is to draw hot air and lint out of the dryer, this lint can become out of control and create a thick buildup in the vent as well as in the workings of the dryer itself. If enough of the lint is coming out of your dryer vent that you notice it, it is an indication that there is too much lint in the dryer vent and it needs to be cleaned out promptly and effectively, which means it needs to be done by an air duct cleaning professional.
An air duct cleaning professional will see excess lint in your dryer vent is a problem for two reasons. The first is that this buildup reduces the efficiency of the machine. You may not actually see the lint buildup in your dryer vent, or see it escape into your backyard, but you may notice that your dryer doesn’t seem to be getting as hot as it used to or that your clothing is coming out still damp. This is caused when the lint in the dryer vent prevents the air from circulating properly, cutting down on how well the machine is able to dry the clothes. This means that it uses more energy trying to operate, and you will have to run each load through more than once just to get the clothes dry.
The other, more serious, concern of having a dryer vent that is filled with lint is the potential for a fire. The buildup of lint can ignite when a spark from the dryer hits the lint, creating flames with plenty of fuel. These flames can then move very quickly through the walls of your home and cause extensive damage before you even notice that the fire has started. An air duct cleaning professional can thoroughly remove the buildup of lint from the dryer vent and the workings of the dryer itself.