When you newly insert a modifier into the modification pipeline its parameters are initialized with the built-in defaults or most recent settings. This may make it necessary for you to adjust parameters over and over again if you use a certain modifier on a regular basis.
To avoid this, OVITO provides the option to save the parameters of a modifier (or combinations of modifiers) as named presets for future use. Modifier presets are preserved across program sessions and allow you to quickly access often-used modifier settings or combinations of modifiers (e.g. to first select a subset of particles and then delete them).
The "Save modifier preset" tool button, which is indicated in the screenshot, lets you define a new modifier preset based on one or more modifiers already in the current pipeline. The current parameter values and settings of the selected modifiers will be saved under a user-defined preset name. The preset will then appear in the available modifier list, following the standard modifiers of OVITO. From here you can quickly insert the customized modifier(s) into the pipeline in the future.