Because the brightness, colour and contrast of computer monitors can be changed, our images can look different on various screens and devices. Colour calibration involves measuring and correcting your monitor to a known state. This means that it will reproduce your images in the same way as your camera, which is also calibrated to this known state.
Although there is no way to know what the screen your audience will be looking at it on will see, calibration allows us consistency across our images, whether on screen or printed. It is always a good option to examine your images on as many devices as possible, to get an idea of what the average is.
Calibration can be done through a combination of hardware and software.
There are software only solutions built into computer operating systems but they are not as effective or accurate as a hardware system.