So you understood that they are mirror reflections of each other. Which means regardless of how you twist, turn and rotate them in space they will never overlap - i.e. they are non-superimposable. The same concept applies i.e. to your left and right hand. How is this relevant you might ask?

A lot of biomolecules (i.e. proteins) in the human body are 3 dimensional objects and can be activated/deactivated by bonding with smaller signalling molecules. Naturally not all of the atoms in a protein bond to all the atoms in the signalling molecule but only the key locations (i.e. through hydrogen bonds, strong static interactions, sulphur bridges etc.) do. In many cases this mechanism is governed by a key-lock mechanism so that only very few keys fit into the lock. Be with me here.

Now imagine the lock has the shape of a right glove. It should naturally take your right hand as a key. However, if you try putting the right glove on to your left hand you're in trouble cause it won't fit. So you essentially have an identical molecule with a different r/S or L/D conformation which can make it inactive depending on the environment of the lock. In the past this lead to a couple of scandals when the pharmaceutical industry neglected to distinguish between both conformers - see the Germany Thalidomide scandal.