In quantum mechanics, fidelity measures the wavefunction overlap between the actual state and the ideal state (i.e. it is the trace of the density matrix). In practice, fidelity measures how close the state or operation is to the ideal version that we write down mathematically. For example, the fidelity of a two-qubit gate operation roughly measures how often you get the correct outcome - a fidelity of 99.9% means the operation completes successfully 999/1000 times. We were the first in the industry to reach 99.9% fidelity.