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An address bus is (part of) a computer bus, used by CPUs or DMA-capable units for communicating the physical addresses of computer memory elements/locations that the requesting unit wants to access (read/write).
The width of an address bus, along with the size of addressable memory elements, determines how much memory can be accessed. For example, a 16-bit wide address bus (commonly used in the 8-bit processors of the 1970s and early 1980s) reaches across 2 to the power of 16 = 65,536 = 64K memory locations, whereas a 32-bit address bus (common in today's PC processors) can address 4,294,967,296 = 4G locations.
In most microcomputers the addressable elements are 8-bit bytes (so a "K" in that case is equal to a "KB", i.e. a kilobyte), while there are also many examples of computers with larger "chunks" of data as their minimum physically addressable elements, notably mainframes, supercomputers, and some workstation CPUs.
See also
"Address bus"