Secondary lint trap
Your clothes dryer performs best if the vent to the exterior is short and lint free. Shorter vents are only possible if the dryer is placed against an outside wall. Lint free dryer vents are only possible by regular cleaning of all lint screens and lint traps.

WARNING Lint not caught by the dryer's lint screen will end up in the dryer vent. A lint build-up can get overheated and is a potential fire hazard, see United States Fire Administration (USFA) statistics below.

It is not always possible to install the clothes dryer next to an exterior wall and the moist air and lint will have to travel a longer distance before it can be expelled to the exterior. The problem with longer runs is that the dryer has to work harder and is less efficient. A secondary lint trap is usually required on longer dryer vents, often in conjunction with a dryer vent booster fan further down the dryer vent.
This secondary trap catches any lint that is not caught by the dryer's lint screen. The lint trap has a clear viewing window to monitor the condition of the diagonal placed filter.

HOW OFTEN DO I HAVE TO CLEAN THE LINT TRAP?
That depends on how often you use the dryer but I would suggest to clean the lint trap as often as you see a lint build-up. However, I strongly recommend to check the secondary lint trap after each time you do use the dryer.
The secondary lint trap will require less frequent cleaning if you clean the dryer's lint screen after each load. It will also be a good thing to clean the entire dryer vent system once a year.

TIP - WARNING If you never see lint in the dryer lint screen or secondary lint trap, check behind the dryer. The vent/dryer connection may have come loose or disconnected.

HOW DO I INSTALL A SECONDARY LINT TRAP?
Cut the dryer vent and remove (cut off) a length of vent as instructed by the manufacturer. Vacuum both ends before putting in the lint trap. Tape up all joints with aluminum duct tape. Do not use regular duct tape because it cracks when it gets hot.