This poem is very interesting, and a bit hard to understand, so I can see why you are having troubles. One of the main ideas behind the poem is that if you are able to rise tall, even in difficult circumstances, you will find joy. She starts off by stating questions that we all have about life: "What is our innocence, what is our guilt?" She says that innocence and guilt are the same, because "all are naked; none is safe." We are all exposed. Then, she mentions that courage, "in misfortune, even death, encourages others." So, when we see someone who shows courage die, we are encouraged ourselves to be courageous. Then, when we see courage defeated, it "stirs the soul to be strong."
She then states that as we are here on earth, the person who "accedes to mortality and his imprisonment" and "rises" anyway, despite circumstances (like a sea rising out of a chasm), finds the strength to continue. This is like a bird, who "as he sings, steels his form straight up," standing tall and singing even though he is a bird who is "captive." So, if we too are captive to our circumstances, do the same; rise up and sing about "how pure a thing is joy."
I hope that breakdown helps a bit; good luck with the poem!