Preset is named set of processing options. Presets can be used in advanced URL format to get shorter and more readable URLs.
Preset definition looks like this:
%preset_name=%processing_options
Processing options should be defined the same way as you define them in the advanced URL format. For example, preset named awesome that sets the resizing type to fill and resulting format to jpg will look like this:
awesome=resizing_type:fill/format:jpg
Read how to specify your presets to imgproxy in Configuration guide.
Preset named default will be applied to each image. This is useful when you want your default processing options to be different from imgproxy default ones.