The Bloom effect  lets you emphasize bright areas (light) or dark areas (shadow) or both in photos or videos, depending on the setting. You can find the effect  in theToolbox under the Objects tab in the Live effects section .

To use the effect, you can  drag it directly onto an image in the Timeline.

You can also drag the effect to the Timeline and later insert one or more images into the effect.

After you insert the image, a colored frame with the title of the effect appears around the small preview image in the Timeline. When you click the colored frame, you can specify the settings for the effect in the Properties area.

In the Properties area  you can now define the settings of the effect.

With Radius and Strength you determine how strongly image areas of the type Light, Shadow or Light and Shadow are overdrawn.
 
To animate the effect, Radius and Strength can be animated with curves over the effect duration. You activate the curve mode at the right edge of the effect.