What does the footnote mean?
The 'width' of the address bus (i.e. the number of signals (bits) that comprise it) determine how much memory can be addressed. Hence, if you have a 16-bit address bus, you can represent a number (in binary) up to 2^16 (=65536) possible values (actually the range used is 0-65535), so you can individually refer to 65536 different memory addresses, or 64K of memory*.
(* This is actually not strictly the case; when people refer to '64k of memory' they mean 64k bytes (8-bit values) of memory. If you've got a 16-bit address bus (so 65536 possible address) and a 16-bit data bus, you can still address 65536 different physical memory locations, but each one is a 16-bit value. As a 16-bit value can also be considered as two 8-bit bytes, you can address 128k(bytes) of actual memory with this setup)