According to the Bronsted Lowry concept, Bronsted Lowry-acid is a substance that donates one or more hydrogen ion in a reaction and Bronsted Lowry-base is a substance that accepts one or more hydrogen ion in a reaction.
Or we can say that, conjugate acid is proton donor and conjugate base is proton acceptor.
The given equilibrium reaction is,
Here, is loosing a proton, thus it is considered as an acid and after losing a proton, it forms which is a conjugate base. That means, are act as a conjugate acid-base pairs.