All plugin control widgets, whether they are created by Ardour or by the plugin, have a common set of controls at the top of the window. These include 4 controls for managing plugin presets.
A preset for a plugin is simply a saved set of values for all of a plugin's parameters. If you load a preset, you are restoring all the parameters of that plugin to the values stored in the preset. This is an easy, fast way to manage your preferred settings for particular plugins.
The preset selector (1) is a regular selector that can be clicked to display a list of all known presets for this plugin. This will include presets that have been created by the user, and for some plugin formats, presets that come with the plugin itself.
Clicking on the preset selector pops up a menu showing the names of all available presets. Clicking on the name of a preset loads it, and various controls in the plugin editor change to reflect the new value of some or all parameters.
Saving the current plugin settings as a new preset is done by clicking on the Add button (2) at the top of the window. A dialog will appear asking for a name for the preset.
To modify the settings in an existing preset, the preset selector must be used to load the preset, then, when the settings have been adjusted, the Save button (3) clicked. The new values will be stored, overwriting the previous version of this preset.
Deleting an existing preset is achieved by loading the preset first, then clicking the Delete button (4). The preset will be removed, and the preset selector turns blank, showing that no preset is currently loaded (although the settings will stay as they were).