Reconnaissance is the act of gathering information. In military/wartime situations, reconnaissance referrs to gathering information on a (potential) enemy's troop strength, movements, environmental conditions, or other desired details. Many reconnaissance operations are covert.
In aerial reconnaissance, aircraft are used to gather information as desired. These aircraft are generally specially designed for low detection or enhanced observation capabilities, but may also be as simple as a single fighter aircraft, depending on the mission requirements. They are usually unarmed or only armed for self defence.