Homocysteine is an amino acid, a molecule with that specific combination of an amine group (NH3+) with an acid group (COO-). Homocystein has also a sulfur atom (S) which gives it its very special property.
Homocysteine, although always present in our body, does not exist in food. It is a by product of the metabolism of methionine, another sulfur bearing amino acid present in the diet. The problem with homocysteine is the molecule is a potent oxidant that has to be removed constantly from our system and the removal is efficient only with a sufficient level of vitamins of the B group, in particular vit B6, B12 and folic acid.