Connecting two CMOS transceivers on different boards
They are series source terminations which are used to match a driver impedance to the transmission line (trace) impedance. A resistor can be placed in series with the transmission line at the driver. Since these are transceivers and bidirectional, they are placed at both ends. Google 'characteristic impedance', 'transmission line' and 'termination schemes'. Basically, you want the driver plus the series termination resistor to equal the impedance of the trace (typically 50ohms). So if the trace is designed to be 50ohms, and the driver impedance is known to be 27 ohms, then a termination resistor of 50ohm - 27ohms = 23ohms is needed.