The TSU-8 CTCSS Tone Decoder is a small board that plugs into your radio. It allows you to search for repeater tones so you can talk. I am not going into what a tone is other than say your radio will transmit a specific tone (you will not hear it) which is recognized by other radios and allows you to use their system. The tones are used in many different ways with radios however, repeater access is the most common.
Look at your local repeaters on line and you will see the frequency and next to it a "tone" number like 110.9Hz, or 156.7 Hz (normally just the numbers) for example. To use that repeater you must enter the correct tone in your radio or the repeater will ignore your signal. You may want to Google it for a technical answer.