Prior to accessing any port in 8255, you must program it with a control word.

This statement may be confusing, but 8255 needs to know which port is to be used as IN and which will be OUT and also the mode in which I/O will operate. All these are programmed with a control word.

This control word needs to be programmed once, every time the 8255 is powered-up. After that, you may use the ports directly. If any change in I/O configuration is required, you need to progrma new control word.
I remember, 8255 ports are all Input after power-up.