I'm assuming dissymmetry means asymmetry here.
Symmetry in chemistry is a lot like symmetry in mathematics. If
a certain molecule is identical to its mirror image, then it is
said to be symmetrical.
Asymmetry means the opposite. An asymmetric molecule is not
identical to its mirror image.
Our hands are an example of asymmetry. Look at your two hands.
You will see that they are mirror images of each other. However,
they are not exactly the same. Whereas, the thumb on the left hand
is on the right, the thumb on the right hand is on the left.
In chemistry, an example is 1-bromo 1-chloro ethane. If you draw
its structure in the tetrahedral shape, you will see that 4
different groups are attached to the second carbon. Now draw its
mirror image. Are they the same in 3D? No.