If you've never had muk (or mook, nokdumuk) before, then it's a little bit hard to imagine. Muk is the consistency and look of american jello, but it is not sweet at all. It has very little flavor itself and takes on whatever sauce you pour on it or serve it with. In korean cuisine, it's usually a flavor-filled soy-based sauce. Muk is made of bean powder, and it can be described as green bean jelly, mung bean jelly, or mung bean starch on a package or in a store. You can buy green bean powder in any korean grocery store, and will also probably find it many general asian grocery stores in the states.