A proxy card takes the place of an original magic card in your deck. Using proxies in place of original cards makes it so that you can keep your originals in perfect condition.
A proxy card is a ‘stand in’ for a card. In other word it’s a replica or replaces a card in your deck. There can be many reasons to use proxies. For example you might want to use a proxy to play with so you don’t damage a super high value card. If you have a $300 card or even a $30, you don’t want to risk bending it right?
Another common reason that folks buy proxies is because they are on a budget. If you want to make a competitive deck, you need dual lands, fetch lands, and other extremely OP staples. These cards cost tens to hundreds or even thousands of dollars. Unfortunatley, many people are priced out of competitive play without proxy cards.
A third reason is play testing. Again, MtG cards are expensive. It’s nice to be able to experiment with a bunch of different high value cards and see what works before dropping loads of cash on original cards.