CTCSS, or Continuous Tone-Coded Squelch System, is a low-frequency tone that other radios can hear. Many amateur radio repeaters use CTCSS to limit the amount of interference. Basically, if a repeater has CTCSS enabled, it will ignore any incoming transmission that does not have the proper tone. You can quickly set CTCSS on the UV-5R in order to communicate with one of these repeaters.
You can set a Transmit CTCSS tone on the UV-5R, so that your transmissions carry the coded tone. This is the most common for repeaters. You can also set a Receive CTCSS tone, which forces your radio to ignore incoming signals unless they carry the proper tone. This is less common, but can be useful if you only want to hear specific messages.
Remember – Frequency Mode is completely separate from Channel Mode. If you want to store a channel with CTCSS, different steps apply.