qubit

An ordinary computer stores information in bits. Each bit is either a 0 or a 1, with nothing in between. A qubit is the quantum version of that unit, and it behaves differently: NIST defines it as the binary unit of quantum information, one that can store a coherent superposition of both binary values at once.

That does not mean a qubit hands you two answers for the price of one. What it gives you is a much larger space of states to work in. When several qubits become entangled, their combined state can no longer be described one qubit at a time, and it is that shared behaviour, together with superposition, that gives a quantum computer computational power an ordinary machine does not have.

The catch is fragility. The same sensitivity to its surroundings that lets a qubit hold a superposition also lets stray heat, light, vibration and magnetic noise disturb it, and a qubit knocked out of its state has lost whatever it was carrying. Most of the hard engineering in quantum computing traces back to that single fact, which is why so much effort goes into detecting and correcting qubit errors rather than simply building more qubits.