In 1887 a Warsaw eye doctor was about to publish a small book proposing a new language for the world, and his heart was hammering. So he wrote to it, in the language itself - eight lines telling his own heart to calm down - and printed the poem in that first book. You are singing the founding document's stage fright.
The verse teaches the negative imperative in its most natural setting: e - do not beat restlessly - and later the reflexive u, calm yourself. Notice too the apostrophes: ', ', l' o. Esperanto poetry may drop the final -o of a noun, and sung Esperanto does it constantly; your eye should learn to read kor' as koro without slowing down. The full rules of that apostrophe - what may be cut, and what never may - are in poetic Esperanto.
The words have been public property since 1917. The tune here is modern: the duo ĴomArt kaj Nataŝa wrote it and released their setting under a Creative Commons licence that asks only for their names - so here they are, with our thanks.