Glop is an unidentified blob of some damp, sticky, or slushy material. When you're walking down a city sidewalk, every once in a while you have to avoid stepping in some glop.
If you don't like the looks of your oatmeal, you might describe it as glop, and your cat's stinky wet food is definitely glop — there's a distinctly distasteful implication to the word. You can also use glop to describe maudlin, overly sentimental poetry or movies: "I couldn't stand to watch that glop, so I turned off the TV." Glop came along in 1943, its sound imitating a ladleful of gummy gunk hitting a plate.