This poem usually gets referred to by its first line: "The Splendour Falls on Castle Walls," or just as "The Splendour Falls." The first line of the poem is kind of weird, though. As we point out in the "Detailed Summary," the word "splendour" is being used in an unusual way. It usually means brilliant or gorgeous appearance, but it can also mean brilliant or very bright light. Having the first word of the poem (okay, the second word after "The") be a familiar word used in an unfamiliar way can feel a little unsettling.
So why might Tennyson have wanted to make the reader feel unsettled or out of place right from the beginning? As we read on, we learn that the valley is somehow magical—it's connected to "Elfland." It could be that the unsettling title is meant to put us on our toes for some ethereal, Elvish magic. The valley also inspires the speaker to think about some deep, philosophical questions (what remnants of our life survive after us? in what ways do the memories of deceased loved ones persist?), so maybe the unusual title is supposed to get our mental wheels turning, too.
Finally, this line gives us the image of something intangible—sunlight—coming up against (falling on) something that is very solid: a castle wall. In a way, this really sums up the poem's central question: how does something like an abstract, intangible memory (or echo) persist in our physical world? The echoes of the poem, like the sunlight, are subject to dying out and disappearing, and their passage out of our physical reality seems to be something that the speaker is very interested in tracing.