SWR (Standing Wave Ratio) is a measure of how well your antenna is "matched" or "tuned" to operate on CB or Amateur frequencies either in your home or mobile in your car. Unlike TV or Scanner antennas that do not transmit, the CB & other "transmit" antennas need to be tuned to operate or transmit correctly & effieciently.
If severely "out of tune" (or having a poor SWR reading), damage to the transmitter can result. While most good CB antennas, when mounted & grounded correctly as the maker intended, will usually have a "safe SWR", it is always still a good idea to check & optimize your SWR reading. Regardless of Mfgr's claims, there really is no way to "Factory pretune" mobile antennas for CB frequencies. The same exact antenna can actually give different SWR Readings when mounted on different vehicles, or even in different spots on the same vehicle! This also applies when setting up a homebase antenna. Location is very important especially when you are close to television aerials as this might cause interference on your own television or on your neighbours television.
Most CB/Amateur radio users have their own SWR meter as they are handy to check your SWR on a regular basis.