The DepthOfFieldEffect simulates a camera lens by blurring parts of a scene not in focus. Distant objects can be blurred or this effect can be used to focus on specific parts of a scene, like an item in an in-game shop.
Like other post-processing effects, DepthOfFieldEffect will only work while Enabled and when parented to Lighting or Workspace.CurrentCamera. Also, it may render differently on low-end devices or depending on your Studio settings (see the Quality Level settings in Rendering → Performance).
For more details on this effect and others, see Post-Processing Effects.