Imagine a penny balanced on its edge. The slightest vibration, wind, or even sound will knock it off its position and force it to land on either heads or tails. This is what a qubit in superposition is like. When the qubit in a superposition it is in a fragile state and can easily collapse to the 0 or 1 state and lose the computing magic of superposition. This is called decoherence. Things that can cause this include light, magnetic fields, heat, radio frequencies, vibration, and other things. This is why quantum computers are built within cryogenic chambers that are close to absolute zero with heavy shielding to keep out even the earths magnetic field and radio waves. Even the act of measuring the qubit will cause it to collapse.
Decoherence is one of the major things that makes quantum computing so hard and is limiting progress. Much research is directed towards error correction schemes and other ways to combat decoherence. The amount of time that a qubit stays in superposition mode before decoherence are routine measured and may only last a few microseconds. The goal is to keep a qubit in superposition mode long enough so that the quantum computer can finish its calculation.