What does "a score" mean in the poem "Loveliest of Trees, the Cherry Now"?

Now, of my threescore years and ten,Twenty will not come again,And take from seventy springs a score,It only leaves me fifty more.

The word score means twenty. "Score" is not commonly used anymore, but it is not entirely obsolete. In the first line of the stanza quoted above, the poet is estimating that his lifetime should be threescore (sixty) years plus ten, or seventy years. In the second line he reveals that he is now twenty years old; so in the third line, when he says, "And take from seventy springs a score," he means he is mentally deducting twenty years (a score) from his seventy years of life expectancy.
Cherry trees blossom very early in the spring. The poet is calculating that he has fifty springs of life expectancy left. It is a nice way of paying tribute to the beautiful cherry trees. He is thinking only of how many times he can hope to see them in bloom again. Many people will agree with him that the cherry tree is the loveliest of trees. They are not only beautiful, but they are harbingers of spring.
The English poet A. E. Housman, who is remembered for his cycle of poems A Shropshire Lad, was born in 1859 and died in 1936; so he actually lived to be seventy-seven years old, a little longer than he had expected.