Calibrating your monitor means:
- If you have two monitors, images will look similar on both
- if you share an image with someone else, they are likely to see something similar to what you see on your monitor (assuming they have calibrated their monitor)
- If you print to a calibrated printer the prints will look as similar as possible to what you see on screen
The best way to calibrate a monitor is to use specialised calibration hardware from Datacolor or X-rite or others, but you can do a fairly good job using facilities built into Window 10. Tim Keane has made a brief guide to this process HERE.
For more on Colour management here are some links
See the page on Colour Management.