Basically, quantum gates are mathematically represented as multiplying your state by unitary matrices. Unitary matrices are ones where their inverse is their own conjugate transpose (i.e. XX*=I). Since each gate has an inverse, for any computation you can reverse it (in other words find a circuit that recovers the inputs from the outputs).

In practice this means that you sometimes have to keep around extra bits called Ancilla bits that just contain garbage, but are necessary for reversibility.

Generally all quantum computing operations have to be reversible at the gate level, so you can’t have a simple thing like an AND gate, and this can make things more complex. As you either need to put some out put to a qubit you don’t care about, or find some other way to handle it.