The glow that spills out of bright things: the halo around a street lamp at night, or the way a highlight smears when a camera looks at something too bright for it.
It happens in three steps. First the effect finds the bright parts of the picture and throws everything else away. threshold sets how bright a pixel has to be to count, and softKnee softens that cutoff so areas near the line fade in instead of snapping on.
Then it blurs what survived, spreading each highlight into a halo as wide as radius. That blur reads the picture at up to 64 points arranged in a spiral, which spaces them evenly enough that the glow comes out smooth rather than showing its own sampling pattern.
Last, the halo is added back on top of the original picture. Adding, rather than replacing, is why bloom only ever brightens a picture and never darkens it, and why intensity above 1 blows the highlights out.