Experiences. Babies are learning an incredible amount each day during their first years of life. It is important to expose them to different environments, sounds, textures, tastes, smells, etc during this time to allow them to learn and experience the world. Taking them outside is great- they love looking at the natural light, plants, hearing the noises of the town and watching other kids play in the park.
Learning. Babies learn by being exposed to different stimuli. To a baby, everything is new, so therefore everything becomes a learning experience. This doesn't mean that you have to purchase expensive toys or items, though. Talking, singing and just interacting with your baby is all the stimulation that he/she needs.