Baby spinach is spinach picked young, with small, tender leaves and a milder flavour. That is why it can be eaten raw, in salads, without any cooking.
I use it fresh in salads and bowls, blended with nuts and oil into bright green pestos, or sautéed for a few moments as a side dish or to enrich grains and fregola. It shrinks a lot when cooked: a generous handful becomes just a few spoonfuls.