What does the poet describe in the first verse of the poem?

"Loveliest of Trees" is written by A.E. Housman and is a poem that both celebrates nature and has a sentiment of the importance of appreciating it while one has the chance.
The first stanza reads as follows:

Loveliest of trees, the cherry nowIs hung with bloom along the bough,And stands about the woodland rideWearing white for Eastertide.

In this stanza, the narrator is providing the imagery upon which the rest of the poem hinges. The cherry trees, which the narrator finds the most beautiful trees of all, are in bloom. This provides a spring setting for the poem, and spring is often symbolic of new beginnings or rebirth. The narrator provides personification for the trees, as they are seen "wearing white" (also a nice example of alliteration) for Easter. All those white blooms are hanging along the bough of the tree, and the narrator imagines that they are doing so to celebrate Easter, which is also symbolic of new beginnings and rebirth. This also provides a spiritual context for the poem, giving a nod of respect to the Creator of those lovely blooms and the beauty of the cherry tree in accordance with Christian Easter beliefs.
After this stanza, the narrator continues on to consider how fortunate he is to witness such beauty and that he needs to take advantage of his remaining years to appreciate the lovely world around him.