The speaker says that "fifty springs are little room" to watch "things in bloom" in spring. The "things" he talks about in particular are the cherry trees, which burst into blossom in early spring.

The beautiful white blooms—"like snow"—that blossom on the cherry trees last only a short time, at most about two weeks, with the bloom at its height for about four to seven days each year. This beauty is fleeting, and the speaker calculates, given that he is twenty now and will live to seventy, that he has only fifty seasons left to see the cherry blossoms.

The speaker is playing with the idea that fifty years seems, on the surface, like a long time. But in fact, he is saying that life moves very quickly. Something like seeing the cherry boughs "hung with snow" might seem like something he has an endless opportunity to do, but actually the window is small, and if he doesn't pay attention, he could miss his opportunities to witness this beauty.

By talking about the cherry trees and then expanding the idea that life is fleeting to all "things in bloom," the poet plays on the theme of carpe diem, the idea we need to live consciously and enjoy what we can in the time we have. Even though nature's bounty seems endless, the speaker wants to be aware that it will pass him by if he is not attentive.