Quantum decoherence describes how a quantum system stops behaving quantum mechanically once it interacts with its environment. Every stray photon, air molecule, or vibration that touches a particle carries away a tiny bit of information about its state, and that leakage destroys the clean superposition the particle was in.
This is why superpositions are so hard to see in everyday life. A single atom can be kept in superposition in a carefully isolated lab, but the moment it bumps into the outside world, decoherence sets in almost instantly, and it starts behaving like an ordinary, classical object.
Decoherence is also the biggest engineering obstacle in building quantum computers. Qubits have to be shielded from heat, vibration, and stray fields, because any leak of information to the environment ruins the calculation before it finishes.