Bloom is a lighting property that shows the "leakage" of light onto or around an object (such as when the glow of the sun comes through the clouds in the sky). The higher the bloom, the higher the leakage. It is like modeling a lens effect.
Each area can have a different Bloom strength, which is set in the Environment schemes. (Note: Bloom used to be set on the Area Node via the Area panel). (need more info on what different values do, and image of what bloom effect looks like)