Complex sentence: The use of frame fields and spin connections are needed in the formulation of a generally covariant fermionic action, (see the article spin connection for more discussion of this), which couples fermions to gravity when added to the first order tetradic Palatini action.
Split into simpler sentences: The use of frame fields and spin connections are needed in the coupling of fermions to gravity (see the article spin connection for more discussion of this). The generally covariant fermion action couples fermions to gravity when added to the first order tetradic Palatini action.