This is part of the seed, usually associated with the embryo nutrition, feeding it during its development. Endosperm is normally a soft, gelatinous or liquid substance inside the seed.
Coconuts, for example, have this "water" inside that is actually its liquid endosperm. Malabar Chesnut (Pachira aquatica) have a weird jelly endosperm (when toasted, it becomes solid and can be eaten without that awkward sensation of chewing a sticky tasteless gelatin). Most of the corn seed is made of its solid endosperm.
Lots of seeds lack endosperm, though. Generally, those are seeds that canīt be stored for much long, because without endosperm, they would need to germinate quickly to get its own food through photosynthesis.