Sometimes an anime is edited when it is dubbed (translated) from Japanese to English. These edits sometimes include things such as blood, violence, swearing, or sex/nudity, but also sometimes include changes to the script, music, and message of the show. The uncut version is the version truest to the show as it was seen in Japan, with all the original artwork, scripting, and music.
Adult Swim shows uncut Anime on TV, while Nickelodeon and CW4kids show it edited. Uncut versions of anime can be bought on DVD or seen online (such as on Youtube), but sometimes the uncut version is only available subtitled.