It will scatter pixels under your finger, they may be all of the selected color, or they may be of a defined range. The amount of pixels, and how often they are scattered can also be fully changed.
You may know it by the name spray on other art softwares, but on pixly it's so powerful that it needed a better name to suit what it actually does, so that's why scatterer
Also known as amount, it's how many pixels will be spilled by the scatterer each time it decides to paint.
This decides how often the scatterer will paint during continuous strokes. Think of this as "How much does my finger have to move before the scatterer will paint again". 0 is immediately, 50 is after a big while.
These are how much will that parameter variate from the current color. This values count both up and down variation. E.g.: If my selected color has a hue of 130°, and I set the Hue Variation setting to 20°, then the scatterer will scatter colors with hue between 120° and 140° ( which is 130° +- (20°/2) ). The same rule applies to the other parameters.
Setting the all variation to 0 will make the scatterer spit only pixels of the color of your selected color.