Additional info can be found here -->
What is a Bidirectional Coupler
A Directional Coupler is an RF transmission coupling device to separately sample the forward (the incident) and the backward (the reflected) wave in a transmission line. Directional Couplers are passive devices which couple a specific portion of the power travelling on a transmission line from one port through another. Directional Couplers are very useful, they enable power levels to be sensed without direct contact with the transmission line.
A Bidirectional Coupler is a four port device. The ports of a Bidirectional Coupler are named:
Port 1 - the incident port or input port
Port 2 - the transmitted port or output port
Port 3 - the coupled port or forward coupled port
Port 4 - the isolated port or the reverse coupled port
When monitoring the Voltage Standing Wave Ratio (VSWR) using a VSWR - or SWR meter (NOT a SWAR meter) of the load at the transmitted port (Antenna), both the coupled port and the isolated port outputs are used to sample the incident and reflected power, respectively.