Kawase blur effect
This is a faster version of the traditional "Blur" effect. It's recommended that you use this one rather than the "Gaussian" blur effect.
Increase the Blur property to make the image blurrier. Higher Quality gives a smoother result but costs more performance, so keep it as low as looks acceptable. The Pixelize X and Pixelize Y properties can stretch the blur more horizontally or vertically.
A common use is to blur the game behind a pause or menu screen: add the effect to the layer and increase its blur with events when the menu opens.
Reference
All effects are listed in the effects reference page.