Drug metabolism is generally classified in two phases, termed Phase I and Phase II.
Phase I reactions include oxidation or reduction reactions, usually through the actions of cytochrome P450 oxidative enzymes or reductases.  These enzymes prepare very lipophilic molecules for Phase II reactions by creating a conjugation site, often a reactive group such as an hydroxyl group.
Phase II reactions “conjugate” a water soluble entity such as acetate or glucuronate onto the drug at the newly created or pre-existing sites, forming  a more polar and water soluble metabolite that can be more easily excreted in the urine and/or bile.