It's the finite field with 2^32 elements. There are several equivalent representations, but we typically work with it as the polynomials with coefficients modulo 2 modulo an irreducible polynomial of degree 32, e.g., x^32 + x^7 + x^3 + x^2 + 1. For more, see [1].

[1] https://en.wikipedia.org/wiki/Finite_field_arithmetic