Plug-in add-on, what is?

What is a plug-in? More of the times of 'other part', the 3th part, like document the factory that you get your software is the 1st par, you the consumer are the 2nd part, and the 3rd part is another factory that makes a software that works with the software that you use, giving more functionalities, I actually use with my film editor 2 plug-ins, one that makes letters to overlay the film, other that are transitions and filters and others visual modifications, and also highlights the letters that you make elsewhere isn’t for typing. Add-ons commonly or only are of the own factory that you get your software and gives you more features, like more transitions, more texts fonts, more filters more sounds, had some that are VFX, like spell hits, guns shoots, and also some that are the thing that a studio would record to be the overlay in video. Rute Bezerra de Menezes Gondim