The mirror image of an orange limonene molecule has the exact same chemical composition, but smells of lemon.
A chiral molecule twists in one direction or another, like the thread of a screw.
Most biomolecules in nature are chiral (sugars, DNA, proteins, amino acids, steroids).
Human proteins are exclusively built from L-amino acids which implies that all receptors are chiral.
That is why mirror image molecules can have radically different activities in the body.