A chiral carbon, (also known as an asymmetric carbon) is a carbon atom which has 4 different atoms or groups of atoms attached to it. For each chiral carbon in a molecule there are 2 optical isomers. This increases exponentially with more chiral carbons, so a molecule with 2 chiral centers would have 4 optical isomers, and a molecule with 3 would have 8 optical isomers. This can be calcultated by 2n where n is the number of chiral carbons, as there are two ways the the atoms can be arranged at each chiral centre. In nature often only one optical isomer is produced, for example only L-isomer amino acids are produced in translation. However the D isomer is dominant for monosaccharides.
The two enantiomers can't be superimposed onto one another as they are mirror images of each other. The difference between L isomers and D isomer the direction that they rotate plane polarised light. This is why they can be called optical isomers. L isomers rotate light anti-clockwise (L for laevo or left) and D isomers rotate clockwise (D for dextro or right). A mixture containing equal parts of both enantiomers is call a racemic mixture and has no net rotation on plane polarised light. If a monosaccharide has multiple chiral carbons, the D or L at the beginning of the name tells you about the carbon that is the furthest from the carbonyl group.