Your trucks allow your skates to turn when you apply pressure to one side (edge) of your skate.
Trucks have a 'pivot pin' which sit in the pivot cups of your plate, and are attached to the plate by the kingpin. Your wheels attach to the axles of your truck.
You should learn how to adjust your trucks to your preferred tightness. We explain it in the video above if you don't know how to do this. You will want to change the tightness on your trucks as your skill level or interests change - make sure to learn how to do so!
You can start with tighter trucks, and slowly loosen them to increase manoeuvrability.
Trucks that are too loose turn easily but are hard to control and will increase instability. Trucks that are too tight will be very stiff and turning might be hard. Trucks that are uneven can create the feeling of your skates pulling in one direction, so make sure they are all adjusted evenly.