You may not have heard of the term before but by virtue of the fact you are here on Instagram you will have likely used the end result of Colour grading. Basically Instagram filters are prepared templates that have been created through the colour grading technique. The 3 colourful circles you see in the picture are what a video editor will use to colour grade video. photo editors tend to have sliders but the concept is the same. Each circle/slider represents shadows midtones and highlights. Ever wonder why shadow areas in all the batman films have a blue tint? Or the Harry Potter films use lots of blue/green hues? Colour grading. Grading is very clever stuff and will be used to both styalise a film/photo and also create a feeling. Colours evoke emotion and well graded films/photos will accomplish this. I dont have enough space to write about this massive subject here but understand that grading has become an integral part of the editing process and it's something you probably use regularly without even realising. It's really worth looking into further and perhaps start exploring this subject.