A fancy slider for the parts fan
The settings allow you to limit the speed of the fan post-slice. These settings only apply to prints run through OctoPrint (they don't work when printing from an SD card attached to the printer). These allow limiting of the fan without having to re-slice the gcode.
The minimum speed is useful if your fan doesn't work below a certain threshold, the maximum setting is good if the fan is too strong and your hot end can't keep up. The default speed is the speed the slider will default to when loading the UI.
The notification timeout setting (not pictured) dictates how long notifications are displayed. When the user sets a speed which is outside the set range via the slider, a notification will appear informing the user that the value was outside the range. These notifications aren't triggered by prints, they only apply to speeds set via the slider.