SWR Standing Wave Ratio, It tells you how your antenna is matched to the transmitters frequency. CB runs on a 11 Meter Frequency. Now we could nevr run a 11 Meter antenna on our trucks that would be a little over 36ft So we use quarter wave whips at 9ft cause we can transmit on quarter half and full waves even 5/8 wave antennas, Most use a loading coil or a fully loaded whip like the firestik, The longer the antenne the better so the 5ft 6ft and 7ft firestik will perform better, But the best tested 5/8 loaded antenna is the wilson 1000 But nothing can beat the 102" whip with a spring.
The better your match the better the performance of your radio. Your radio is limited to 4 watts so if you have a match at 2.0 your now losing I think half your power, and 4 watts is not much to even start with. I would not even key up a radio if the match is over 2.0 You start reflecting that power back into your radio and becomes heat and burns out whats called the finals in the radio, Once you blow those the radio is dead. Most radios have a built in SWR meter, If not even a cheep radio shack SWR meter will help you tune your antenna.
A simple tip to remember to know if its to long or to short
If the SWR is higher on channel 1 than it is on channel 40 the system is electrically short.
If the SWR is lower on channel 1 than it is on channel for the system is electrically long.
There is more to confuse the heck out of you, Coax length, So if you buy a firestik and use there coax, stick to 19ft, you get RG8x you now have to use 14.25ft, a thing called velocity factor, its how fast radio waves will carry over your coax.