What is a re-entry permit?
If you have a non-immigrant visa (possibly one that has been extended for a year) it will be cancelled if you leave Thailand.
So to avoid your visa being cancelled, get a re-entry permit from your local immigration office.
This is very important if you have a work permit, because canceling your visa also cancels the work permit and you have to start the process all over again.
If you have a multiple entry non-immigrant visa which has been extended on a work permit (or you have a work permit application in process) then you still need a re-entry permit, because a new entry is considered to be a new visa, and everything will have been cancelled.
If in doubt, check with immigration first, because the consequences of getting it wrong are troublesome.
