In a literal sense, the poet is saying in the first two stanzas of the poem that, since he is already twenty years old, he probably has at best only about fifty more years to live. That being said, he laments in the last stanza the fact that he will see the cherry trees in full bloom only fifty more times, which "are little room", or not very much. When looking at the poem literally, the word "snow" can be taken in a literal sense as well. The poet decides to make the best of the situation by taking the opportunity to enjoy the trees even in the winter, when they are "hung with snow". The cherry trees do not have to be blooming for him to appreciate them; they have great beauty even in their branches are bare.
Metaphorically, the poet is expressing the ability to appreciate life in all its stages. Life is of course wonderful and beautiful when it is new, when it is "wearing white for Eastertide", a time symbolic of renewal and rebirth. The time of freshness for new life, however, is limited, both for the poet as a living being, and for all things in nature. The poet seeks to convey a message of positiveness and optimism even so. As he grows older, using up his allotted time of about seventy years, things will not be as easy and lovely for him as they are when he is in his prime. He will no longer possess the beauty of his youth, and hardships and ailments, "snow", may beset him. Still, the poet determines that he will still be able to see the joy and beauty in his life, just as he can see and appreciate the cherry tree in the winter, when it is not in bloom, but "hung with snow".
Check out the link below for a much more in-depth analysis of the poem...
Housman lived in England where the soft, sweet flowers of the cherry trees in spring provide a brief, sensual experience that is almost painful in its intensity. After the long, drab English winter, we have this one beautiful explosion of joy before the winds blow the petals away.
Housman’s poem recreates this feeling in those of us who know the drifts of “snow” created by the fallen cherry flowers in a way I have only seen in cats rolling in catnip. It is a physical experience, not a coldly analytical one.
The idea that we should be one day deprived of this experience is appalling. The briefness of the cherry bloom is a reminder of the briefness of our own lives.