Click on the Pokestop to receive free items such as Pokeballs, Greatballs, Ultraballs, Revives, Potions of all kinds, Berries to feed to Pokemon and even Pokemon eggs to hatch as you walk!
If you ever see a Pokestop with pink flower petals coming off of it that means it has a lure on it. Lures are given to you when you level up or buy them from the shop. With a lure, you can place it on a Pokestop and what it does is attract Pokemon to that location. Other players can also benefit from your lure and you can benefit from any lure someone else places of course.