You can think of your radio's mode as the method by which your voice (information) is sent to someone on the other end. There are many ways to manipulate radio frequencies to send information, selecting the right one depends on who you are talking to. You always want to match your mode with the person on the other end, with AM being the most common for CB radios. To help you understand a bit of what's happening, in AM mode, your voice is being "carried" on top of the RF carrier. Think of the RF carrier as the car, and the modulation the people in it. Other modes, like single sideband (USB/LSB) are more efficient in specific instances, but will require more technical know-how from the radio operator. The most common mode for anyone travelling down the highway (like truckers) will be AM mode. Updated in 2021 CB radios can now have FM as a mode. You can view FM CB Radio here
Why you normally use AM mode most of the time
AM stands for Amplitude Modulation. This means when you talk, you are changing the amplitude, or amount, of your RF carrier wave. AM is preferred because it is easier to use, since you do not need to adjust controls like your clarifier during normal operation.
How to determine which mode to use.
AM is the default mode for most standard CB communications. If in doubt, set it to AM