The Freeform tag is optional - it lets you tell the person who writes for you something additional that you might like in this fandom. For example, I might like "threesomes" in one of my fandoms, so I would input that. You don't have to input anything if there's no particular kind of fic you want there. It's mostly concepts like that - "AU", "Crossover" "Genderswap".
Optional details are where you write a little bit about what you would like in a fic. For example, I put for one of my fandoms, "So ridiculous, so seriously heroic. My favourite characters are Faiza (Excalibur), Jacqueline (Spitfire) and Meggan (did Gloriana stick?) but I love the whole damn lot of them. Even Captain Midlands. I would love a fic about their adventures, or their downtime, or sniping at each other when they're meant to be working."
But you can just leave it blank if you want, too, or just put that part in a Dear Yuletide Author letter, if you have a lot to say. It's just to give your author more options or inspiration.