for questions about special graphical effects loaded by OptiFine or special shader mods, not anything in Minecraft Vanilla

Shaders are used to enhance or completely replace Minecraft's lighting system or make even bigger changes to rendering.

Not to be confused with the simpler graphical effects used when spectating e.g. creepers or endermen, included in Minecraft Vanilla.

Please also use the tags and on questions with this tag.
The warning of applies here as well: If you game crashes or otherwise does not run properly anymore while using shaders (also called "technical issues"), the question is off-topic here.