Chirality means that there is a mirror image of a molecule that cannot be twisted into the original shape, despite being structurally identical. Due to the particular ways molecules tickle each other in living organisms to do interesting things, that means that the mirror image (racemate) of a molecule does something different.

In chemical synthesis, most (but not all) processes tend to preserve chirality of molecules: replacing a bunch of atoms in a molecule with another set will tend to not cause the molecule to flip to a mirror image. If you start from an achiral molecule (one where its mirror image can be rotated to the original), almost all processes tend to end up with a 50-50 mix of the two racemates of the product.

In biochemistry, you can derive all of the amino acids and sugars from a single chiral molecule: glyceral. It turns out that nearly all amino acids end up in the form derived from L-glyceral and nearly all sugars come from D-glyceral. The question of why this is the case is the question of homochirality.

There's as yet no full answer to the question of homochirality. We do know that a slight excess in one racemate tends to amplify into a situation where only that racemate occurs. But we don't know if the breakdown into L-amino acids and D-sugars (as opposed to D-amino acids and L-sugars) happened by pure chance or if there is some specific reason that L-amino acids/D-sugars is preferred.