Stupid question

Your engine is water cooled, that is the engine coolant expansion tank. If you are topping it up it should be with Triumph recommended coolant or distilled water in an emergency. With the engine cold the coolant should be about level with the min line in the bottle, when its warm it will get higher.

 

Don't get too worried about using Triumph recomended coolant but there is a couple of kinds for older and newer bikes somewhere like Halfords should be able to help as long as you avoid the usual Kevin the teenager type (who usually can be found around the car stereo section)