This feature allows you to use Virtual Studio Technology DLL plugins to apply effects to the audio. To use this feature, select the VSTs tab from the Options window, where you can type or browse for the directory on your computer that contains VST plugins - note that the plugins must be DLL files.
Once you have specified a directory name, you should see a list of VST plugin names appear in the effect's tab under VST Effects. Select the plugin you want to use and double-click it to make it appear in the Effects list in the middle panel. When the VST effect is selected from here, its controls will show in the right panel, where you can adjust the effects parameters.
Also please visit http://www.kvraudio.com, which is a comprehensive information resource for all types of plugins, including VST and DirectX plugins.
To see a list of recommended free VST plugins, visit Free Vst Plugins.