Can someone explain to me the Meter/stress/rhythm used in this poem?

One thing I'd add is that often the most important part of meter is where it changes. Note that you have a strict pattern of iambs--four to a line (tetrameter) in lines 1-6. Line seven begins with a stressed syllable: "TREE and FLOUR and LEAF and GR" followed by a line with a "spondee" (two accented syllables in a row) LET them P, LET them P. The pattern continues through all three stanzas, with minor variations which I would chalk up to Tolkien's inadequacies as a poet rather than to an attempt at deliberately interrupting the meter.