Secondary UAP Ethernet ports don't provide PoE passthrough (to run current to a second powered device), but they do support data passthrough. It serves as a bridged interface between main / secondary Ethernet port.
This means users can connect the PoE cable to the main Ethernet port to supply power, then connect another, non-PoE device (e.g., IP camera, extra UAP) to the secondary Ethernet port to bridge connections.
The exception to this is the UAP-AC-IW and the UAP-AC-IW-PRO models which do allow PoE Passthrough through one of their secondary ports. You will see them labeled as "PoE OUT" for PoE passthrough and "Data" for only data passthrough.