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.
Common uses are blurring a background layer behind a menu or pause screen, or creating a depth-of-field / focus effect. Because it's cheap to render, you can also animate the blur strength with events (for example, progressively blurring the game while a menu opens). This is a 2D-only effect.
Reference
All effects are listed in the effects reference page.