It's a battery inside your computer that saves your cmos settings in the event of an extended power outage. The CMOS keeps the time on your system and when the battery starts to go, the first indication is an inability to maintain the correct time. Computers used to use "AA" batteries but nowadays most use the flat CR?? type batteries. You can look in the user manual for your system or just open the box and look for a battery. Don't remove the old one until you are ready to install the new one (you can take it out for a moment to look at it if you need to).
BTW, if this is not a new problem but has been happening all along, it's possible you have the wrong time zone set in the Date & Time applet in the control panel. Thus, when time sync sets your clock it's setting it to the zone set rather than the correct zone.