DEFINITION: Shutter speed, to put it simply, is the length of time that your shutter opens, allowing light to enter the camera through the lens to expose your sensor (or in those rare cases, your film).
In your camera there are set durations that your shutter will open. In the “olden days” your shutter speed was controlled mechanically. In modern SLR’s, the shutter is controlled electronically, allowing for more precise control and more specific lengths of time. Control over your shutter speed went from full stops with mechanical shutters to 1/3 stop increments with an electronic shutter.