What is a hydrophobic patch?

Hint: hydrophobicity is the physical property of a molecule that is repelled from a mass of water (known as a hydrophobe), but there is no repulsive force involved; it is just the absence of attraction. Hydrophilic molecules are those molecules which are attracted to water.

Complete solution:
Hydrophobic molecules are nonpolar and, thus, prefer other neutral molecules and nonpolar solvents to dissolve whereas water molecules are polar; hydrophobic molecules do not dissolve well among them. If Hydrophobic molecules are mixed with water they often cluster together, forming micelles.
Hydrophobic patches are defined as clusters of neighboring apolar atoms which can be accessible on a given protein surface; they have been investigated on protein subunit interfaces.
Hydrophobicity is an important determinant of the structure and function of proteins as it is the driving force behind the folding of soluble proteins and when exposed on the surface, it is majorly involved in recognition and binding of ligands and other proteins.
Hydrophobic patches contain a large fraction of polar/charged atoms and they have chemical compositions similar to the more hydrophilic protein patches.
Studies have shown that the largest or second largest patch on the accessible surface of the entire subunit was involved in multimeric interfaces in 90% of the cases of the protein patches,these results have proven to be useful for subunit design and engineering as well as for prediction of subunit interface regions.

Note:
Interactions between proteins are a part of numerous biological processes and are essential for the proper functioning of the cell. The importance of hydrophobic residues in driving protein interactions is universally accepted, a characteristic of protein hydrophobicity, which informs its interactions, has remained elusive.