Create material with Custom / New UI Widgets / UIFlareTransparent or Custom / New UI Widgets / UIFlareGlobal shader and set it to
UIFlareGlobal works only in Render Mode = Screen Space - Overlay.
Color of the flare.
Size of the flare in range [0..1].
Speed of the flare relative to the image size: how much times it will move from left to right in one second.
Delay between appearances of the flare. Examples:
0 - no delay
1 - delay is travel time from left to right
2 - delay is doubled travel time from left to right