Out-of-Band authentication factors are identity signals that do not rely for their veracity on the same system requesting user authentication.
For example, an SMS code sent to a mobile phone to enable a desktop login is likely to be an out-of-band identity factor. However, an SMS code sent to a mobile phone to enable an app login on the same phone is generally not an out-of-band identity factor, since proof of identity is being requested in order to use phone features, yet the identity "proof" being offered is possession of the very same phone.
Such "in-band" authentication flows make compromise relatively easy if the phone has been stolen. Out-of-band authentication factors are generally considered to be significantly stronger proof of identity than in-band authentication factors.