As there is a weekly cap for earning guild reputation and Blizzard makes sure you are aware of it every time you turn in a quest, this addon was written to reduce the number of warnings you receive.
The first time you receive the message (for each game session), the message is displayed as normal. However, any future times the message would show up, the message is automatically hidden so you don't see the warning.
(So yes, technically "NoGuildReputationWarning" is incorrect, as it really is "SingleGuildReputationWarning", but meh...)
UPDATE: Supposedly a hotfix Blizzard did in the last couple days has made this addon unnecessary. "After hitting the weekly guild reputation cap, players will no longer receive the message that they're at the cap every time a quest is turned in."
- Updated string matching to use Blizzard's variable rather than a set string (should make this locale independent)