An effect is a postprocessing effect.
Each frame, once the 3D scene has been rendered, postprocessing can be applied to it. It’s effectively a stack of shaders that take in the rendered 3D scene image, and transform it before it is rendered to the screen.
Postprocessing can get computationally heavy quite quickly, as each postprocessing pass requires the whole screen to be rendered.
This is especially true to effects such as blur or bloom, which require each pixel to sample the neighbouring pixels multiple times.