You probably noticed that the title of the poem – "Beat! Beat! Drums!" is also the first half of the first line. Sure, poems do that sometimes, taking on the first line as their title. But wait a second: in this case, that line repeats at the beginning of the second and third stanzas, too. So there's definitely something important to it.
This repetition sets a sort of rhythm or tempo for the poem (and since it's in free verse, it doesn't have that otherwise). In fact, it's kind of like a drum beat. Oh, poetry, that's why we love you.
Of course, the words themselves mimic the sound of a drum. They're one syllable words, so they sound kind of like the one syllable rap or thump of a drum. And their forcefulness is accentuated by those exclamation points. So, even before we get into the poem, when we still, don't know what the drums are for, the title lets us know these drums are powerful, and that they're creating some major excitement.