A qubit is the smallest unit of information in a quantum computer – comparable to the classic bit in normal computers. While a bit can either have the value 0 or 1, a qubit can be both 0 and 1 at the same time. This phenomenon is called superposition.
In addition, qubits can be entangled with each other, so that the state of one qubit is directly linked to that of another, even if they are far apart. These properties enable quantum computers to perform certain calculations much faster and more efficiently than classical computers.