HC11 does I/O and other "extracurricular activities" through various ports. Each port is an 8-bit, or 1 byte, port. Each bit in the port corresponds to a real pin on the CPU, and thus the value of that bit (1 or 0) corresponds to a real voltage level on the circuit board.
The HC11 does what is called "memory-mapped I/O". That is, the "registers" that contain input data are accessed by loading a value from a particular memory location, and the way to output a value is to store it at a particular memory location.
The HC11's "register block" is at $1000 to $103F. By the name "register block", the HC11 documentation means "the area of memory that is not actual memory, but I/O ports and configuration registers". Look at pages 38 and 39 of the pocket reference for the details on the register block.
Each link below gives more specific information for some of the ports.