In simple terms, remote sensing is the act of detecting objects in an area and measuring their movements from space.
Remote sensing detects and monitors the physical characteristics of the ground by measuring its reflected and emitted radiation from a distance. The sensors used for remote sensing are generally mounted on satellites, aircraft and more recently, drones. The data collected from these sensors span a variety of wavelengths of the electromagnetic spectrum. Common wavelengths are visible light wavelengths which include red, green and blue. Data collected from the visible spectrum can create images of the ground that are easy for the human eye to interpret. Think of these images like pictures of the ground taken from a very powerful camera, high above the surface of the earth.
The images from remote sensors come in a variety of resolutions, including multiple spectral bands and can be used for many different purposes.