What is Thomas Randolph's "A Song" about?
Thomas Randolph's "A Song," is a poem about nature.
The speaker calls on Music—personifying it, as if it could create music on an instrument)—and asks that it ("the queen of souls") play a song on its "powerful lute" (a musical instrument), but asks for a sad "requiem" (which is a Mass said in the hope that a soul that has passed on will rest peacefully).
However, the direction and action of the poem changes when the speaker then asks Music to 'run gentle, nimble hands' over the strings to change the tempo and mood of the sounds it creates so that even the forest will come alive: the trees of various kinds (cedar, elm, palm, myrtle, and oak) will dance in a "jolly train" or line.
Until, and here the tempo and mood of the poem changes a second time, a sad note is played and the trees become "fixed" and unmoving once again.
By the way, the poem is written in rhyming couplets: pairs of lines are joined that have the same rhyme at the end of their lines ("string" and "sing").