Memory mapping is a process whereby some item of digital hardware is connected to a processor's address bus and data bus in such a way that it can be accessed (for reading and/or writing) exactly as if it were a memory cell.
This is used as an alternative to connecting it to an I/O port, especially in embedded systems.
For example, an analog-to-digital converter could be memory mapped to a certain address. When that address is written to, the conversion is started; when the address is read from, the data is transferred to the processor.
Sometimes only partial address decoding is used, meaning that the device effectively occupies a much larger block of memory space than is strictly necessary. This can be a problem if the memory space is small (e.g. with a 16-bit address bus which can only address 65,536 different locations).