I like both of the answers that are posted above. Since there are many ways to interpret a poem, and since Cummings is especially open to personal interpretation, I'll offer my thoughts.
We usually think of love as an overwhelming experience, something that knocks us off our feet. A crush, an infatuation, or, most famously, a Romeo and Juliet type of love-at-first-sight. But Cummings has something else in mind with “somewhere i have never travelled,gladly beyond.”
The first time you read a poem, don't try to analyze it for anything. Just read it. In fact, read it a second time the same way. Then start looking for things—language and devices that the poet uses to convey his central message, his theme. One good way to analyze a poem is to look for diction patterns. Diction is word choice. Let's look at a common thread of diction that Cummings has chosen to include in this poem:
silence, frail, gesture, cannot touch, near, slightest, carefully, intense fragility, texture, breathing, small hands.
Cummings is using words like these to convey the idea that the love he is talking about is subtle. The object of his love is able to skillfully open and close his innermost being, like peeling back the petals of a flower, then closing them again. It's so much more than the explosive, all-enveloping experience that we normally call love. It is the ability of the loved one to open us up to something new, something we could not even see about ourselves.
Which brings us to the poem's last line (remember that this looks grammatically incorrect because Cummings writes this way on purpose):
nobody,not even the rain,has such small hands
Throughout the poem Cummings has referred to himself as a rose that can be opened and closed by his loved one. To take the metaphor a small step further, let's think about why Cummings mentions rain in the last line, for the first time in the poem. What do roses need to survive? Water, of course. In fact, once they have sprouted and grown, that's pretty much all they need. But water can also be deadly to something as delicate as a rose if it is not received in the correct quantity. Too much rain will kill any land-based plant. It takes the right amount. The rain's “small hands” are, as noted in a post above, individual raindrops. These raindrops are what the rose needs to flourish. The speakers love interest has something even better than the rain's small hands, in terms of her ability to affect him. It it the finer qualities in her eyes, her gestures, her slightest look, her intense fragility, that enables her to open and close him, to realize his true self, which is something he cannot do without her.