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