"Above the fold" placements place your creative in a location on the page that does not require the user to scroll down. In other words, when a user lands on the page that contains your creative, your creative will be located somewhere in the top portion of the page. Above the fold placements typically have better clickthrough rates and are ideal when you want to capture the user's attention as soon as they reach the page.
"Any available" placements will display your ad anywhere on the page. If you're looking for maximum volume, "Any Available" is the best option because it will match the most broadly.