Threshold
A cut-off: brightness above it becomes one thing, and below it another.
See it in Dither engine. Under Image processing, move Threshold and watch which parts turn to dots.
In plain words
A threshold is a line you draw through a range of values. Everything above the line is treated one way, and everything below it another. Set it at half brightness, and light pixels turn white while dark ones turn black. Tools use thresholds to decide where edges are, which pixels to sort and when a glow should start. Moving the threshold changes how much of the picture crosses the line.