@Mohsin *******
You received a 60-days stay permit stamp in your passport when you entered visa-exempt. When you exit Thailand after 30 days, your 60-days stay permit will become invalid. So when you want to re-enter Thailand visa-exempt, you again will get stamped in for a 60-days stay permit. But you cannot repeat this infinitely. At some point after two or three visa-exempt entries, the border official will pull you aside and ask you about your intentions, to find out if you are a "real" tourist, or if you are misusing the visa-free policy for illegal work or for a longstay. At this point you might be asked for the "3 famous proofs": proof of accomodation, cash 20,000 THB or equivalent, and an onward travel proof out of Thailand within 60 days