I bet you can't wait to scroll down to the comments to tell me I spelled it wrong. But, unfortunately for you, this is how they spell it in the official NATO phonetic alphabet. Because not every NATO country speaks a language with the "ph" digraph.
Anyways, in the signal flag alphabet, this blue and white one stands for "A". It's also one of 2 signal flags that isn't a square.
Alfa is used by diving boats to signal that they have a diver in the water. This tells boats around the area to be careful - slow down and avoid the area if possible. If not, keep an eye out for dive flags/buoys and don't run on top of them.
If used together with number flags, Alfa is used to signify azimuth or bearing. Basically, it can be used to tell another craft which way you're heading, so they can avoid you. GPS, radio, and all sorts of other fancy computers usually cover those things nowadays, but if your boat's power goes out this is a great backup system.
Check out the rest of our Signal Flag explanations!