The L is for Levo, meaning that the substance rotates the plane of polarized light to the left. If there is an
isomer, the opposite (
mirror image) would be the D (for Dextro) form.
When chemists whip up a batch of an amino acid, they may well end up with roughly half of each orientation, whereas all known forms of life on this planet can only (efficiently) make or use the L orientation.
It is one of the mysteries of science why this L preference occurred back in the goo stages of evolution.