What was Big Willy Shakes going for?
We don't think Othello is talking about high school graduation here. So what's the guy saying?
First, we should tell you that pomp comes from a Latin word (pompa) which means procession, or something with a bunch of ceremony, grandeur, and sometimes even arrogance. (Side bar: ever heard someone called pompous? It's the same root word.) Pomp's partner in crime (circumstance) draws from the idea that fuss that happens around a particular event.
So we might read this as saying "a bunch of fuss at a ceremony" or "a huge display at an event." Sure, it's a little redundant pairing two words with similar meanings, but Shakespeare did it, and it stuck. The phrase does kind of roll off the tongue after all.
Now that we know what the words mean, let's see how they fit in this scene. Iago is ripping apart Othello's marriage and life, and Othello just wants everything to end. If his wife is cheating, his life might as well be over. Check out how he says "farewell" to his old life on the battlefield, commanding armies. He thinks that life is full of "pride, pomp, and circumstance" because there's so much glory and ritual around warfare.
Othello thinks all that war stuff is his whole life because he doesn't know anything else. He's shaken up at the news (or, lie, we should say) that his wife is cheating on him because he thinks it means he's a bad guy and terrible soldier, too. He wants to flush his life down the toilet.