Assuming that by ‘subobject’ we mean (an equivalence class of) monomorphisms, this means that for every object , the (generally large) preordered set of monomorphisms with codomain is equivalent to a small poset, or equivalently that this preordered set is essentially small. Variations exist that use notions of subobject other than monomorphisms.
If is well-powered, we say that is well-copowered (although “cowell-powered” is also common).
Relation to local smallness
A well-powered category with binary products is always locally small, since morphisms can be identified with particular subobjects of (their graphs).
Conversely, any locally small category with a subobject classifier must obviously be well-powered. In particular, a topos is locally small if and only if it is well-powered.
There are interesting conditions and applications of the preorder on the sets of subobjects in well-powered categories, cf. e.g. property sup.