Secondary UniFi Access Point (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 "in-wall" 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.
To find what speeds the port support, please check the Networking Interface item of the device's Specifications table on the Datasheet or Quick Start Guide, downloadable in our Downloads page.