Without chemical bonds, the universe would be nothing but a thin soup of individual atoms drifting alone through space. No water, no air, no rocks, no life — just isolated atoms. Chemical bonds are the forces that hold atoms together, and they come in three main flavors. Covalent bonds form when atoms share electrons (like two friends splitting a pizza). Ionic bonds form when one atom transfers electrons to another (like one friend buying the other a pizza). Metallic bonds hold metal atoms together in a "sea" of shared electrons. Each type creates substances with vastly different properties.
Why do bonds form in the first place? It comes down to energy. Atoms bond because the resulting molecule or compound has lower energy than the separate atoms — and nature always favors lower energy states, just as a ball rolls downhill. The energy you would need to break a bond apart is called the bond energy, measured in kilojoules per mole. Triple bonds (like in nitrogen gas, N₂) are stronger than double bonds, which are stronger than single bonds. Nitrogen's triple bond is so strong (945 kJ/mol) that N₂ is remarkably unreactive, making up 78% of our atmosphere essentially unchanged for billions of years.
Chemical bonds determine virtually every physical property you can observe. Diamond and graphite are both pure carbon, but diamond has each carbon atom bonded to four neighbors in a rigid 3D lattice (making it the hardest natural material), while graphite has carbon layers bonded in sheets that slide over each other (making it soft enough for pencil lead). Water is liquid at room temperature because of hydrogen bonds between its molecules, while carbon dioxide (a similar-sized molecule) is a gas because it lacks those bonds. Understanding chemical bonds means understanding why the material world behaves the way it does.