Okay kiddo, do you know what an accent is? It's a special mark that goes over a letter to show you how to say it. For example, in the word "café" the little accent over the "e" makes you say "ca-fay" instead of "ca-fee."
Now, a double acute accent is when there are TWO of those little marks over a letter. It looks like this: ő . It's used in some languages, like Hungarian, to show that a certain vowel is pronounced differently than it would be without the accent.
So, let's say you see the word "kő" in Hungarian. If that double acute accent wasn't there, you might think it's pronounced like "ko" or "kow." But with the accent, you know to say it like "kay-uh". It's like adding an extra hint to help you say the word correctly. Pretty cool, huh?