The power output of CB radios is limited by law. Often, your radio will come already adjusted to the maximum power output. Amplitude Modulation (AM) is the most standard form of CB radio communication, and it is limited to just four watts of output power, which is measured by the antenna connector on the radio. The other option is single-sideband (SSB) transmissions. These offer a lower audio quality than AM, but they allow you to communicate over longer distances. The FCC limits the output to 12 watts of envelope power, measured at the antenna connector.
Remember, these power limits are imposed by law, meaning that it is illegal to use an external amplifier to boost the output. Also, you cannot modify the radio to exceed these power limits. If you violate these power output limitations, this can lead to fines, equipment confiscation, and even imprisonment.