A tri-folder is an A4 leaflet that folds two times into three-thirds of the A4 page. You can also refer to it as a 6pp DL because it results in 6 panels on a single leaflet. You can design on all of the six boards, creating a mini brochure. Tri-folders are cost effective as they offer you a multi-page mini booklet, which is printed out of a single sheet.
You can create a tri-folder in two different ways. One of them is forming a Z fold, and the other is creating a roll fold. A Z fold is generated when the leaflet folds back in equal portions similar to a concertina effect. The roll fold, on the other hand, is created when panels inwardly fold over each other.
Tri-folders help you to achieve several things. One of them is effectively reducing the entire size of your leaflet while still keeping the space. You also get the chance to use the fold of your own choice to showcase your design style. Tri-folders have a tactile as well as visual sense, and this is something flat leaflets cannot convey.
Here is one of many tri-folder I have done