Useful phrase originating in a series of infuriating advertisements for Ronseal varnishes. To paraphrase the ad:

Ad man: Ronseal quick-drying five-year woodstain is a quick-drying woodstain from Ronseal, that lasts for five years. Ronseal. It does exactly what it says on the tin.
As you can imagine, there are endless possible permutations of this advertising formula, for each of Ronseal's self-explanatory products.

Although "Does exactly what it says on the tin" is a registered trademark of Ronseal, it is widely used to indicate self-evidence or redundancy. e.g. "What does the self-destruct system do?", "It does exactly what it says on the tin".