Free-space optical (FSO) communication systems are a non-fiber-optic based optical communication systems that aim to handle large amounts of data at a high bandwidth. The simplest example of FSO is using a flashlight to send Morse code to a neighbor. FSO communication is an optical alternative to fiber networks that can act as a backbone for rapidly deployable mobile wireless communication needs, covering scenarios such as disaster relief response, temporary wireless networks for events, or communication over distances where laying cables is impractical or illegal, such as laying cables on land not owned by the user. FSO communication is particularly helpful where the physical connection of the transmitter and receiver locations is difficult to create, such as mountain ranges or harsh environments.
An FSO system offers many features: low-cost set-up and operation, bandwidth on par with fiber optic cables, light-speed transmission, resistance to signal interception, and license- and regulation-free use.  FSO systems offer a flexible networking solution that requires no license, has a high data rate, is immune to radio interference and jamming, and can be deployed anywhere in the world with proper line-of-sight between transmitter and receiver.
FSO systems were designed to be a lower-cost alternative to fiber-optic communication systems, particularly in places where the deployment of fiber is unrealistic or impossible. Since FSO can receive and transmit through windows and some types of walls, it is possible to mount FSO systems in office buildings right against a window or wall, reducing the complexity in wiring fiber or electronic cables while providing a secure transmitter and receiver for all kinds of applications.  
FSO systems have lower SWaP compared to RF, making them cheaper to launch into space and maintain over time. The typical diameter of an FSO receiver aperture is smaller than an RF antenna, but no so small that it is difficult to manufacture, thus making the receivers cheaper as well.
The frequency for FSO operations is in the Terahertz range, an area which is license-free and very broad. One Terahertz has one million Megahertz worth of frequencies to choose from, and one typical bandwidth of the communication band for optics stretches from 186THz to 199THz, leading to almost 13 million available frequencies just on the Megahertz level. Again, this is only for one communication bandwidth; there are dozens across the optical spectrum depending on the end application.
See the article Interception Resistance.