A watchdog timer is a timer that is used to make sure some unexpected problem does not keep the system processing as it should.   There are several of them in the SLC and they are common to every PLC that I know.  In the most common sense (and probably what you are referring to), there is a watchdog timer that covers the processor scan.  If the scan takes too long to complete due to a poorly written loop or jump routine, the watchdog timer will expire and fault out the PLC.  This is a major fault and is only recoverable by clearing the fault, fixing the problem, and starting the PLC back up.  It's primary purpose is to prevent the PLC from going into LaLa land (technical term) due to a programming error.  You generally will not have a problem with this error.  You can change the watchdog timer period in the case that you have a massive program that needs a long time to process.  Generally that means you have the wrong processor for the job - need more power.  With the SLC there are also hardware watchdogs that look for unexected hardware issues, STI watchdog, and the program scan watchdog that I have explained.