Angiosperms are flowering plants. To start with, pollen allows the sperm to be transported without water. Mosses need water so their sperm can swim (moss sperm is flagellated), and the sperm can't survive being dried out. In angiosperms, the sperm cells are kept safe inside a pollen grain, and the pollen grain itself can be transported in a variety of different ways (e.g. wind, bees, birds, bats, etc.) The flower itself plays a role here, because it can attract pollinators. After fertilisation, the seed shells protect the embryos (seeds contain baby plants in a kind of dormant state), a secondary endosperm provides the embryo with food, and outside the seeds, you're going to have some kind of fruit. Fruits play an important role because they aid in the distribution of seeds. If a bird eats a berry, for example, there's a good chance that the seed will pass out the other end of the bird somewhere else and sprout into a new plant, or the bird might pick up some berries and drop them or even hide and forget them. Mosses don't have this option.
