Notice how the Sun suddenly becomes very, very bright in X-rays before returning back to normal again. The y-axis (the vertical axis) is marked with letters from A to X.
This is simply a code we use to classify the maximum brightness reached by a particular flare.
So a small flare is called an A-class flare and the really big ones are X-class flares.
Don't get confused here with all these 'X's - all flares give off a lot of X-rays. An X-class flare is just the very brightest of all.