Before revision: It is a similar concept to a stub; but, whereas a stub is terminated in a short (or open) circuit and the length is chosen so as to produce the required impedance , the λ/4 transformer is the other way around; it is a pre-determined length and the termination is designed to produce the required impedance transformation.
After revision: The quarter wave transformer is an alternative to a stub; but, whereas a stub is terminated in a short (or open) circuit and the length is chosen so as to produce the required impedance , the λ/4 transformer is in series with the load and its length and characteristic impedance aretransformation designed to produce the required impedance transformation.