You mean (...must have meant) that, given the pde (P) \texttt{L}u=f on an appropriate space, the right hand side belongs to H^{-1}. The most appropriate space for the solutions is, in this case, L^{2}.
The reason for requiring f\in H^{-1} is that now the problem (P) can be put into variational formulation, and then the methods of functional analysis can be applied: Say, for linear problems, the Lax-Milgram Theorem. Or, for nonlinear parabolic problems, monotonicity methods.