Must have visual effects in a game

[Radikalizm]

As a simple basic explanation it is correct that post-processing is a technique of applying an effect or filter on a rendered image, although when you look into it more deeply you'll find that it goes far beyond this

Most post-processing systems are set up so they can take advantage of other data besides the previously rendered image, this is heavily used in deferred renderers where the G-Buffer is exposed to the post-processing pass which can result in some amazing things that can be done purely as post-processing effects (eg. screen-space water, volumetric lighting, etc.)