Series resistor in data lines
A series resistor between a high-speed driver and a trace is to match total impedance of driving source (impedance of driver + series resistor) with characteristic impedance of transmission line (PCB trace). If the source impedance is not matched to transmission line, multiple reflections would occur, such that the reflected waves would go back and forth, and disrupt signal levels at receiver end, vastly reducing signal quality of the channel and killing signal margins.

In modern (good) silicon designs, however, there is no need in impedance-matching resistors, because normal high-speed interfaces nowadays have controlled impedance, and board designers learn to design traces with perfect match. The resistors are used only in poorly-designed silicon chips where the driver impedance control and properly controlled termination is not implemented.