MAC stands for Media Access Control. Basically it is like a unique serial number for your device. Every device that has network connectivity has at least 1 MAC address, including your CSB.
Your BLACKs second screen displays 3 things. The IP address assigned by your router, your router (GW or gateway) address, and your MAC address.
Your MAC address consists of six 2-digit pairs of characters. A character may be a number 0-9 or a letter A-F. MAC addresses may be formatted using a space between the pairs, a colon, or a hyphen depending on the device. The colon is the most common, but we display it using a space for easier readability.
Here are 3 samples:
See also: How to find MAC Addresses, Network Overview