How to use guard in swift instead of if

How to use 'guard' in swift.I have gone through many articles about 'guard'.But i didnt get clear idea about this.Please give me clear idea.Please give me sample output for following 'if' statement.

if firstName != "" 
{
   if lastName != "" 
   {
      if address != "" 
      {
        // do great code
      }
   }
 }

you can use guard as well and your code will be more readable

let firstName = "First"
let lastName = "Last"
let address = "" // empty

if firstName != ""
{
    if lastName != ""
    {
        if address != ""
        {
            print(1,firstName,lastName, address)
        } else {
            print(1,"address is empty")
        }
    } else {
        print(1,"lastName is empty")
    }
} else {
    print(1,"firstName is empty")
}

func foo(firstName: String, lastName: String, address: String) {
    guard !firstName.isEmpty else { print(2,"firstName is empty"); return }
    guard !lastName.isEmpty else { print(2,"lastName is empty"); return }
    guard !address.isEmpty else { print(2,"address is empty"); return }

    print(2,firstName,lastName, address)
}

foo(firstName, lastName: lastName, address: address)
/*
 1 address is empty
 2 address is empty
 */
foo(firstName, lastName: lastName, address: "Address")
/*
 2 First Last Address
 */