Gooey things are drippy and soft, as well as sticky. Your very favorite dessert might be a huge, melty, gooey hot fudge sundae.
Few things are more annoying than stepping on something gooey on the sidewalk, or getting a gooey piece of gum stuck in your hair. Gooey candy, likewise, might taste delicious but get stuck in your teeth. The informal gooey is the perfect way to describe these gummy, tacky substances, and it comes from American-coined slang, from goo, probably a shortened form of the now-obsolete burgoo, meaning "thick porridge."