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 is a regular selector that can be clicked to display a list of all known presets for this plugin. This will include presets that you have created yourself, and for some plugin formats, presets that come with the plugin itself.
Click on the preset selector to pop up a menu showing the names of all available presets. Click on the name of the preset you wish to load. The preset will be loaded — you may see various controls in the plugin editor change to reflect the new value of some or all parameters.
To save the current plugin settings as a new preset, click on the Add button at the top of the window. A dialog will appear to ask for the name of the preset.
If you wish to modify the settings in an existing preset, first use the preset selector to load the preset, then adjust the settings as you wish. When done, click the Save button and the new values will be stored, overwriting the previous version of this preset.
To delete an existing preset, use the preset selector to load the preset. Click the Delete button, and the preset will be removed. The preset selector turn blank, showing that no preset is currently loaded (although the settings will stay as they were).