A differential pair is a pair of traces that together carry a signal between them. Because of the circuitry involved, these traces may be isolated from the surrounding circuit, and thus, it is necessary to keep them near each other. Why? Maxwell’s law says that a changing magnetic field through a loop of wire causes a current. In our cases, the pair of traces is a loop – because they are connected to each other (through circuitry) at the ends of the trace – the changing magnetic field could be from electromagnetic interference, and we are trying to keep a signal “clean” as it travels over our board.
Thus, Cadence has features for creating “differential pair” traces, where the traces are kept together, to minimize the area between them (the area of the loop being an important element of Maxwells equations.)