How do we know if this following molecule is a superimposable mirror image or non-superimposable mirror image ? [See Image attached]
Take 2 round, soft objects like potatoes or apples.
Pin into each 4 pins of 3 different kinds ( 1 + 2 + 1 ),
pointing to the most different directions.
Call the 1st pin bromine, the 2nd one hydrogen, the 3rd one ethyl.
Do the same for the other potato to be the mirror image of the first one.
Try to rotate one of them to look like the other.
If you succeed, they are superimposable.
Now, replace one of the same pins on both objects by another -
4th kind - of pins in such a way that they are still mutual mirror images.
Try to match them the same way you tried previously.
If you succeed, they are superimposable. 


The summary:


While there are 2 hydrogen atoms on a carbon atoms - or more generally, there are only 3 kinds of groups on a C atom - the groups/atoms of the same kind are interchangeble and you can rotate the molecule to match the mirror image. 
If there are 4 different groups on C atoms, you cannot make such a match. 
Said by other way, the former case has a plane of symmetry, the latter case does not.