A garage door strut is a brace that is installed to give garage door sections/panels structural integrity. Without a strut on the top section, you will void your warranty if you connect an operator to it. Struts will make sure the top section doesn't fold in when the motor tries to pick the door up. This strut is a 2" tall.
An 8 foot wide garage door should have one strut installed, and it should be at the top of the top section/panel.
Struts are screwed into the back side of the garage door as seen in the images.