Superposition is a way of describing possibility before measurement. A qubit in superposition is not in both states at once in any everyday sense. It is in a state where the amplitudes for 0 and 1 are both nonzero. When you measure, you get one definite result -- 0 or 1 -- with probabilities determined by the amplitudes. After measurement, the state updates to match what you observed.

Apply the Hadamard gate (H) to $|0⟩$. The result is $\frac{|0⟩+|1⟩}{\sqrt{2}}$, an equal superposition. Run many measurements on identically prepared qubits and you get roughly half 0 and half 1. But each individual measurement gives just one result.

If a qubit is in the state $\alpha$$|0⟩+\beta$$|1⟩$, then measuring in the standard basis returns 0 with probability |$\alpha$|² and 1 with probability |$\beta$|². This rule is called the Born rule. After measurement, the state becomes $|0⟩$ or $|1⟩$ — whichever was observed. This is irreversible: the pre-measurement superposition is gone.