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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)
What does the footnote mean?