The I/O (really 1 and 0, by the way) enables the Cruise.
The + button sets the speed at whatever speed you are going when you press it. If the system is already active, then this button will increase the speed until you let it go.
The button with the speedo symbol is the resume key, it brings the car back to the set wpeed after using the brakes or the clutch.
You drive along, minding your own business, and decide that the cruise control would be nice, you press the 1/0 button -- binary for on/off -- to turn the system on. Then you attain the desired speed and press the + button. Life is grand, the sun shines and the birds sing.
You need to slow for traffic ahead, so you hit the brakes. The cruise control turns off -- goes to sleep, really -- and you slow to whatever is necessary for the conditions. The conditions resume to the pre-slowing state and you speed up as is appropriate. You realize that all is well with the universe, the sun shines and the birds sing, so you press the button with the speedo symbol on it and tak your foot off of the gas pedal. The car remembers the speed it was previously going, and -- wait for it... -- resumes that speed.
Conversely, you are cruising along with the cruise control set, and the conditions change and you need to be going faster for a few moments, whatever. All you do is press the gas pedal to speed up. Do whatever needs to be done, then release the gas pedal and the car will slow to the speed it was already going. The Cruise is still doing 60, while you overrode it to 70, all you do is let off of the gas and the car will return to 60 without doing anything else.
You can also be cruising along with the Cruise Control actively setting your speed, and press the + button and the car will speed up, just like hitting the gas pedal. The difference here is that when you release the + button, the car will maintain the new speed.
The real question here is, how come this needs to be explained? Can't you figure out what the buttons do while pressing them as you drive?
Two things are always true, you can add speed at any time by pressing the gas pedal, and you can turn the system off -- put it into standby, actually -- by pressing the brake or the clutch. If the system is running, you can over ride the set speed by pressing the gas pedal. The car will return to the set speed when you release the gas pedal unless you use the brake or the clutch. If you use the brake or the clutch, the system will be disabled -- put to sleep, not turned off -- until you press the resume button. The car does not have the ability to push the pedals softly -- it will not resume the set speed casually -- so the resume mode will be full throttle accelleration until the previous speed is reached, and then the throttle will be reduced to maintain that speed.
Press the 1/0 button to turn the system off. The default is for the system to always be off, so if you have it on and set, then park the car the system will be off when you start it again. Some systems will turn off if the speed drops below about 30, but some will retain the previous set speed and allow you to resume as long as you have not actively set the system off by using the 1/0 button. (1 = ON, 0 = OFF)