This was one of the terms that I didn't understand when I started making cards and it may be the same for you ..so here is matting and layering de-mystified.
Cardmaking- Matting and Layering (card-making-magic.com)
Related Articles

https://www.youtube.com/watch?v=DXz6Couftt0 #Handmade #Valentine Triple Easel #Card: How to Make #Love …
DIY Valentine Card – How to Make Triple Easle Heart ‘I Love You’ Card

https://www.youtube.com/watch?v=F5qvUhSnUrY There are two kind of vellum, including card stock …