A MAC address, also called Ethernet hardware address, is the unique identifier assigned to each Ethernet interface on your Access Point. MAC addresses consist of six groups of two hexadecimal digits separated by colons.
A single MAC address is printed on the label on the back side of each Access Point. However, Access Points with 2 Ethernet interfaces in fact have 2 distinct MAC addresses. The MAC address printed on the back label belongs to one of those Ethernet interfaces. You can then find the other MAC address by increasing the last digit in the first MAC address by +1.
A PA1200 WiFi Access Point has such a label on its back side:
The MAC address printed on it is 54:9C:27:XX:XX:00 and corresponds to one of its Ethernet interfaces. The MAC address of the second Ethernet interface will thus be 54:9C:27:XX:XX:01.