afaik, having an Equity card means that you are in the stage actors' union. I don't know much beyond that, but most jobs will specify whether they are equity or non, and there are more equity jobs than non, and they pay much, much better. Also you can't just join, you have be offered the card....I think.
You can find a lot of information about Actors' Equity on their site:
http://www.actorsequity.org/