Our world has two types of seeds, endospermic and non-endospermic. The word endosperm means "seed within", so, literally speaking, an endospermic seed would have seed within the seed. A non-endospermic seed, therefore, would NOT have seed within the seed.
As plant embryos develop, they need some source of food. In some plants, tissue is formed that takes up most of the seed and serves as sort of a storehouse for food. That storehouse is called an endosperm. As endospermic seeds further develop, they form embryonic leaves within themselves which feed on the endosperm for nourishment. Wheat is an example of an endosperm.
In non-endospermic seeds, the embryonic leaves (called cotyledons) take up most of the space in the seed and these cotyledons become filled with the material that will nourish the growing embryo. Thus, their food is not within the seed, but within the cotyledon. Peas and squash are example of non-endosperms.