One thing about hides: they come in unusual sizes and shapes. In order to turn the hides into the coverings of the Tabernacle, they had to be cut into appropriately-sized and -shaped pieces. Before this could be done, guidelines had to be drawn into the hide. This is called scoring, the melacha of m’sarteit.
Drawing guidelines on just about any surface in order to indicate where to cut would be m’sarteit. For example, folding a piece of paper on Shabbos to indicate where to rip or tear even after Shabbos is scoring. However, as with all the melachos of preparing the hides, m’sarteit does not apply to food items. Accordingly, one may use a knife to draw guidelines on a cake to help cut the proper size slices. In fact, there is specifically a custom to score one’s challah in order to indicate where to cut after making the bracha; this practice is in accordance with the halacha.
This is just an introduction to the concepts of the melacha of m’sarteit; it is not a substitute for a full study of the halachos.