A NegateOperation is the result of a part that has been negated through Studio's solid modeling Negate tool.
A negated part turns pink and translucent as an indicator of its state. If the negated part is then unioned with a normal part using the Union tool, sections where the negated part overlaps the normal part will be cut out.
Note that you can undo part negation by selecting the negated part and clicking Negate again.
See Solid Modeling to learn more about Studio's solid modeling tools and methods.