The hydroxyl group is a functional group characterized by the presence of an oxygen atom covalently bonded to a hydrogen atom (R-OH). It is also called alcohol group, and the molecules that contain it have in their names the suffix "ol". In biochemistry, this group is particularly important because it can act both as donor and acceptor of H in hydrogen bonds. Therefore, their presence in biomolecules increases their water solubility and hydrophilic character.
This is a non-charged polar group. However, in some biochemical contexts it can function as a weak acid, losing one H+. This behavior is particularly important in the case of phenolic hydroxyl groups, i.e., in the case of hydroxyl groups bounded to benzene rings. As the main example of this situation, we have the amino acid tyrosine, which in some proteins may have an acidic behaviour.