Antenna performance will depend a lot on having a good ground. The BEST way to mount the antenna is to drill a hole in the car body (best radiation pattern on a car, will be mounted directly in the middle of the roof). You, like most people, will probably never do this and will instead opt for a magnetic mount, which will scrape the hell out of your paint and chafe underneath the coax. Don't say I didn't warn you ahead of time.
It works well enough, though.
"Matching" the antenna as it were, is a matter of checking it's resonance on the desired frequency. A properly functioning and matched antenna will radiate nearly all of the transmitted power. If the antenna is not resonant on a given frequency, some of the transmitted power will be reflected back towards the transmitter. Too much power reflected back has a tendency to harm the final transistor(s) in the transmitter, though modern CB radios have protection mechanisms that throttle back the transmitter power when this is detected. Even so, any power being reflected back towards the transmitter is power that is not being radiated by the antenna. In reverse, it also means your receiver's functionality is less than ideal.
An antenna that is at an improper length for the desired frequency is only one thing that can affect it's resonance. A poor ground will do it, defective coaxial cable will do it, poor connections will do it, etc. A perfectly resonant antenna will present a feedpoint impedance of 50 ohms to the transmitter, which along with the coax, also has an impedance of 50 ohms. Anything that causes the impedance of the system to deviate from 50 ohms, will result in some of that transmitted power being reflected. These reflections are called standing waves, and the difference between the maximum and minimum amplitude of the standing waves, are expressed as a standing wave ratio (SWR).
With CB radios, the equipment used to measure amplitude of the standing wave is going to be measuring in volts, so it's properly called the Voltage Standing Wave Ratio, or VSWR. A VSWR of 2:1 means that the maximum voltage of the standing wave is 2x greater than the minimum voltage of the standing wave. A VSWR of 1:1 means that the maximum and minimum amplitudes of the wave are identical, i.e. there is no standing wave (this is more theoretical - there's always going to be something).
To measure the VSWR, you'll need a meter. They're around $30, give or take. You can follow the instructions that come with it, but it's pretty much works like this: After inserting the meter between your transmitter and antenna, you select a frequency in the middle of the range of frequencies you wish to use (say, channel 20 on a CB radio). You then calibrate the meter by transmitting on that frequency with the VSWR meter selector switch set to FWD, and adjusting the meter until the needle indicates maximum VSWR (it's usually denoted with something like "set" at the end of the scale). While still transmitting, you flip the switch over to REF and note the reading on the meter. This is your VSWR.
On a CB, anything at 2:1 or lower is perfectly fine. Some (better) antennas allow you to adjust their length (the length of the antenna changes it's resonance on a given frequency, and thus the feedpoint impedance at that frequency). If you can manage a VSWR of 1.5:1, you're in great shape and can call it a day. People that feverishly labor to lower get it any lower than that on a CB are wasting their time for almost no realized benefit (another topic I'd be happy to discuss, math included, should there be any disagreement).
With a decent, well made commercial antenna (Wilson, K40, etc) a VSWR over 2:1 nearly always indicates a poor installation. Bad coax, poor ground, bad connection, etc.