Fortuitous means by chance, like a lucky accident. If you and your best friend's families happen to go on vacation to the same place at the same time, that’s a fortuitous coincidence!
Something fortuitous is random like an accident, but there’s no downside. A rock falling on your head is an accident, dollar bills falling on your head is fortuitous. The meaning of fortuitous is changing from "happening by chance" to "lucky chance" because people get it mixed up with fortunate. But watch out: If you say fortuitous to mean just plain lucky without the element of chance –– that's a usage error.