These are the lines in the poem which are the most famous, probably both becuase they are kind of catchy, like a slogan or a proverb, as well as their ambiguity. Keats saw this as a weakness in his work and never really gave a particularly satisfying answer. There is the question of who is speaking and to whom, as well as the ambiguity of what truth and beauty refer to in the work. These are answerable with close study, but there's also some room for interpretation (of course, students of this kind of thing rarely need much room to debate every last jot and tittle of the piece!). Truth and beauty, both eternal.
There is a question on this very topic in the Q and A section. I think that the lines help to bring to light the idea of "negative capability." Essentially, this is a belief that Keats advocated in suggesting that there are some realms where human desire to appropriate through calculation and analysis will not be entirely present. In these realms, humans must learn to live with "the unknown" and the idea of embracing this level of not knowing. Essentially, the unknown is cool. This flies in the face of Western rationalism and an embedded perspective to know everything through scientific thought and deductive analysis. Keats response of "that's all ye know and all ye need to know" is fairly profound in this light.
The phrase about truth and beauty provokes and solves an aesthetic conflict in John Keats's "Ode on a Grecian Urn." Timeless, perfect beauty can only exist artificially (in art). Humans cannot remain in this captured truth; therefore, human beings do not need to be perfect since it is not possible for mortals.
It's been a long time since I've read this poem, but the relationship between truth and beauty is in each of the scenes on the urn. For instance, the youth who chases his lovely young woman and they are just on the verge of a first kiss--they are caught forever in that moment of intense joy and excitement, forever young, forever lovely, forever in love. The truth is on their faces and in their body language, and the beauty is caught forever in the etching on the side of the urn for as long as the urn exists.
The same is true for the beauty of the tree, the beauty of the seaside town (of course, we don't know what the truth is there since there are no people in the picture to help tell the story...perhaps a festival that everyone has attended, or the ceremony where the flower-laden cow is being led to sacrifice?), the silent music that is being played which is beautiful to everyone since everyone must imagine its sound for him or herself.
The truth is there for all to see as long as the urn survives, and the beauty is in the art of the pictures and the story behind those pictures.