Think of a proxy as a small-scale "poster" on A4 paper that stands in for a giant billboard. You use the lightweight version for the work and swap back to the original for the final "billboard" export.
Printing on thin paper or sticker sheets and sliding them into a card sleeve in front of a real card for proper weight and feel. How To Make FOIL Proxies For Magic: the Gathering FILMY PROXY
Once imported, you can toggle a "Use Proxies" button in your editor to switch between the low-res "editing" files and the high-res "master" files instantly. Physical "Paper" Proxies Think of a proxy as a small-scale "poster"