If you find yourself in a situation where there isn't enough light to take a good photo you can use a flash to boost the light. You can also use a flash on a bright, sunny day to fill in shadows. This is technique is called fill flash.
A flash is an instant burst of light which helps light up the image. It comes from an electronic device called a flash or flash unit. Most new cameras have built in flashes and on some older models you have to attach the flash to the top of the camera. Hand held battery powered flash units are also available which may be operated remotely with a sync cable.
A flash will only supply enough light for a certain area. An example may be 15 feet. There are stronger and weaker flashes available and a flash meter will let you know how far you can use your flash. In studio photography, flashes are usually big stand alone units or strobes. These can also be operated by special battery packs and synchronized with the camera by cables or radio transmitters.
There are various types of flashes and they start at about $50.