If a substance is hydrophobic, it means that it does not interact well with water and tends to repel it. These substances are typically nonpolar, meaning they do not form hydrogen bonds with water molecules. Hydrophobic materials are crucial in biological systems, for example, in the formation of cell membranes, where hydrophobic tails of phospholipids create a barrier that separates the interior of the cell from the external environment. Understanding hydrophobicity is vital in fields like biochemistry and molecular biology, particularly in studying protein folding and interactions.