To aerate something is to put air into it. When a pastry chef whips cream, she aerates it, filling it with air until it's light and fluffy.
Gardeners sometimes aerate soil with special spikes, allowing air to penetrate the surface and helping lawns or plants get more oxygen. If you aerate water or juice, you carbonate it, resulting in a fizzy soda. Cooking, baking, aquarium maintenance, and chemistry are just some of the activities that occasionally require someone to aerate some material. In Latin, aer means "air."