It's the informal, singular form of the verb "avoir" (to have).
Here are some examples:
* Tu as un chien? (Do you have a dog?)
* Tu as beaucoup d'amis. (You have many friends.)
* Tu as faim? (Are you hungry?)
Remember that "tu" is the informal, singular form of "you", so you would use "tu as" when talking to a friend, family member, or someone younger than you. For a more formal situation, you would use "vous avez" (you have).