For example for spaces of holomorphic functions on the open unit disc, the Hardy space H2 consists of the functions f whose mean square value on the circle of radius r remains finite as r -> 1 from below.
Such spaces have a number of applications in mathematical analysis itself, and also to control theory and scattering theory. A space H2 may sit naturally inside an L2 space as a 'causal' part, for example represented by infinite sequences indexed by N, where L2 consists of bi-infinite sequences indexed by Z.