The re-entry permit is a permit which is added to your passport when you have an extended 1 year Thai visa. Note if you have a multiple entry Thai visa this would not be needed but a visa that has been extended in Thailand for 12 months after showing you met the financial requirements needs a re-entry permit when you leave the country.
The purpose of a re-entry permit in Thailand is to allow you to leave Thailand and return and to continue with the visa where it was when you left. As an example your visa was extended in January 2013 for 1 year to January 2014. Then in April 2013 you leave the country and return a week later. When you arrive at Thai immigration or any entry point into Thailand they will stamp your passport until January 2014. You do not need to start the 12 month visa application again. If you do leave Thailand without a re-entry stamp in your passport and you arrive back in Thailand they will not stamp your passport for January 2014 but only for 30 days.
The re-entry permit is very important as once you leave without a re-entry stamp in your passport you will return to Thailand with a 30 day option only and you would need to leave the country again. Obtain a new 90 day visa and start the entire process from over again. Note that this re-entry stamp is for a 12 month marriage visa, 12 month retirement visa or if you are currently holding onto a Thai work permit.
If you have any questions about Thai immigration and Thai visa then ask us on the chat messenger or see our main website for more details.