When your supporter starts filling out their street address, the form will automatically start to suggest an address to make entering that information that much faster and easier for your supporters. When you select an address, it will populate the Address, City, State, and Zip.
Go into the activity you wish to enable the Address Lookup feature.
Go to the Online Form category in the right and open the Contact Information section of the appropriate form.
Click on the Edit icon for the Street Address.
Enable the switch by Assist filling address info.
Your form should now automatically fill the address as you type it.