This piece of software was created to allow the development of plug-in filters for Adobe Photoshop in more programming languages.
Photoshop is a very popular application, and was one of the first to allow extensibility of its functions by the means of 3rd party plug-ins. There are many functions that a plug-in may perform, but the most common one is image filtering, i.e. a mathematical operation performed on images that results in changes in image appearance, for example blurring, sharpening, and even artistic effects.
The most important limitation of the Photoshop plug-in system is that the plug-ins may only be written in C/C++, and although that would be the natural choice for performance reasons, there are many cases in which an interface with a different language would be needed.
The plug-in host is a wrapper for plug-ins that conform to the ActiveX specification and implement a required interface. Simply put, this means that you can write the plug-in in any language that supports ActiveX output, such as Visual Basic, or any .NET language.