After sifting, the flour was mixed with water to make dough. The combination of a liquid with solid particles to form a paste-like consistency (or firmer) is the melacha of losh. Losh includes both the initial introduction of liquid to these particles as well as the subsequent mixing.
According to losh, one may not prepare such foods as oatmeal and mashed potato flakes on Shabbos, independent of cooking considerations. It may be permissible, however, to mix things such as baby cereal in a thinner consistency, more like a pourable cake batter. Even in such a case, one must employ a shinui (a difference in the act of preparation) in both the pouring and the mixing parts of the process. Anything that results in a pure liquid, such as stirring iced tea mix into a glass of water, is not losh and has no restrictions on its preparation.
Losh is not limited to food. Mixing cement on Shabbos would be violation of losh, as would playing with modeling clay.
This is just an introduction to the concepts of the melacha of losh; it is not a substitute for a full study of the halachos.