Closed Captioning (CC) displays text on the screen and is usually a transcript of the program's audio.
You can access the closed captions setting from TiVo Central by going to: Menu > Settings > Accessibility.
You can also change the setting from the info page of a show. Press Info on your remote and then select the Closed Caption option. In both cases, closed captions can be toggled on or off for all programming.
Note: Keep in mind that not all programs will have closed captioning available but there is usually a CC icon at the start of programs for which it is available.