The nonconscious describes any mental process that goes on in which the individual is unaware. Nonconscious processes are not thought about consciously.
There are two components to the nonconscious. The first is preconscious which is information that is not currently being thought about but can be retrieved easily. Examples of preconscious thoughts are memories or information that isn't thought about until it is primed by a reminder. You may not currently be thinking about that leftover pizza in the fridge but when someone says the word 'pizza' you remember and get excited.
The second component of the nonconscious is the unconscious. These are the processes that we are unaware of like cognition or multitasking. The unconscious is important when driving. You are looking at the road, traffic signs and signals, and speedometer. Your foot is controlling the gas pedal and brakes and your hands are controlling the wheel. And all of this is being done while you can have a conversation with a friend in the passenger seat. You are mostly unaware of the plethora of nonconscious though processes that are going on at any moment.