A bottom-left yellow right isosceles triangle with a top-right red right isosceles triangle make up Oscar, which stands for "O" in the signal flag alphabet. Whew, that was a mouthful, right. Well think of it this way - it's a red and yellow sandwich sliced the big way. This is an important one to remember.
It's important to remember because it means "MAN OVERBOARD!!!". Pretty sure international law dictates the three exclamation points and all caps. Obviously if you see this you should head over and help out in the search.
Two Oscar signal flags are used in Flag Semaphore, a form of communication involving reading the position of flags held by a flagman. This is way more involved and confusing than signal flag shorthand, so if you see someone waving 2 Oscars at you go get someone who isn't me to explain it to you.
Check out the rest of our Signal Flag explanations!