Air is all over this poem, in one way or another. There's the "airman" of the title, the clouds that the speaker mentions twice, and all that stuff towards the end about breath. You could say this is a very, ahem, airy poem. Get it? Anyone? Nothing? So, why is there so much air? The speaker is worried that his breath will just be wasted, so he takes to the clouds, searching for a life that is not a waste. It's as though the clouds offer breath, or air, that is not wasted.
Title: The poem is about an airman, a guy who spends his time up in the clouds flying a plane.
Lines 1- 2: The speaker knows he will die "somewhere among the clouds above." The phrase "meet my fate" is an example of periphrasis, a roundabout way of saying something. Here the speaker says he will meet his fate rather than he will die. While these clouds are just clouds, they are also a battlefield, the place where the airman will die.
Line 12: The clouds-as-battlefield idea is expressed again. Here, they are described as a scene of "tumult," which seems like a bit of an understatement to us. We can definitely assure you that planes shooting at each other and dudes dying is a little more than just a tumult.
Line 14: The speaker says the future ("years to come") seemed like a "waste of breath." This is a metaphor for the fact that the future seems totally pointless.
Line 15: The speaker uses the same metaphor to describe the past as wasted breath. We also have a neat figure here called chiasmus (named for the Greek letter Chi). A chiasmus is a group of phrases that take the form of X-Y-Y-X. So, here we have years to come-wasted breath and wasted breath-years behind.