Heavy baryon chiral perturbation theory (HBChPT) is an effective quantum field theory used to describe the interactions of pions and nucleons/baryons.
It is somewhat an extension of chiral perturbation theory (ChPT) which just describes the low-energy interactions of pions.
In a richer theory one would also like to describe the interactions of baryons with pions.
A fully relativistic Lagrangian of nucleons is non-predictive as the quantum corrections, or loop diagrams can count as
Because the baryons are much heavier than the pions, HBChPT rests on the use of a nonrelativistic description of baryons compared to that of the pions.