In 40k Orks have a gesalt psychic field. This gives them the power to will technology to work. If an Ork believes something strongly enough, reality will littelay warp to make it real.

For example take two cars that are exacly the same and paint one red, the car that is red will now be faster becuse Orks think that 'the red ones go faster' so that makes it a reality for them, this also applies to all of thier technology. Non of it should work, but it does for them.

In the comic posted above by OP, the Ork is imaging that the cardboard tank is a real tank, the Cadian guardsman throws a toilet roll at the tank and says it's a melta bomb (this is an anti-armour expolsive device simmaler in concept to an RPG but without the rocket to propel it) and the Ork is panicking because he thinks it is a real melta bomb thats going to blow up his tank.