The I/O object controls your device's individual I/O lines and ports. Each port groups eight I/O lines. You can control the state of each line or entire port.
The list of available I/O lines and ports is platform-specific, i.e. it depends on your device. Two enum constant sets — pl_io_num and pl_io_port_num — define the set of available lines and ports. You can find the declaration of these enums in your platform documentation (for example, EM1000's is here).
There are two different methods for controlling I/O lines (ports):
I/O lines of your device can work as both inputs and outputs — Controlling Output Buffers topic explains this.
Some I/O lines can work as interrupts — see Working With Interrupts for explanation. The list of available interrupt lines is platform-specific. The pl_int_num enum constant set defines the set of related variables. Again, please refer to your platform documentation.
On many devices, a number of I/O lines may be shared with inputs/outputs of special function blocks (serial ports, etc.). When a special function block is enabled, I/O lines it uses cannot (should not) be manipulated though the io object.