Bouillon, in French cuisine, is simply a broth. This name comes from the verb bouillir, meaning to boil. It is usually made by the simmering of mirepoix and aromatic herbs (usually a bouquet garni) with either beef, veal, or poultry bones and/or with shrimp, or vegetables in boiling water.
This is not to be confused with bouillon soup, a Haitian soup, or stock, which is made in a (somewhat) similar manner, but serves an entirely different purpose in French cuisine.