Okay, I haven't seen very thorough threads on the Render Effects Option present in the game, so I will try my best to explain them, feel free to correct any mistakes I may make.

First off this graphic option is not recommended on slower computers with integrated graphics or generally low specifications.

Render effects basically tweaks graphical effects and particles and fogs to look more better and visually pleasing

These Present effects include:
-Background Depth of Field Present in stage backgrounds to refine distant details. (This one is the most noticeable among most players)
-Weapon Effects Refined Charging certain weapons and releasing the charge is noticeable between render effects on and off, examples include the Barbarous Thorn Blade or Antigua lines, and Wild Hunting Blade wolvers. The spectral creatures and effects of these weapons look a lot better when render effects is turned on, you can see more detail on the creature too, as compared to turning off render effects, it just looks like a mess. (Low Deailed)
-Certain Enemies Look Better Monsters such as slimes have noticeably better transparency effects, and chromalisks as well have better cloaking effects.
-Fog & Particles Refined Fog present on "High" graphic settings looks a lot better along with better particle details and perhaps status effects also May include more details. (Unsure about this as I have not looked into this category much.)

***If anyone has anything else to add to this, just comment down below with what else is noticeably more refined with render effects present.

Hope this helps! Thanks!