The Equal Liberty Principle has a longer history. The idea that everyone should be granted the greatest degree of liberty consistent with similar liberty for others is defended at length in John Stuart Mill’s essay On Liberty. In fact we could take some variation on this principle as the core tenet of Liberalism as a political theory. This principle doesn’t tell us that people should be free to do as they please no matter what. At some points, my being free to do something is liable to interfere with your being free to do something. For instance, my being free to host parties with live bands into the early hours of the morning might interfere with my neighbor’s being free to get a decent night’s sleep. In the interest of maximizing equal liberty for all, we would be justified in restricting people from activities that would interfere with the liberty of others. This has many familiar applications. Neighborhood zoning regulations are one example. A good deal of environmental regulation illustrates this idea. Maximizing liberty for all equally requires that we restrict businesses from being free to pollute where doing so would adversely affect the health of others.
Rawls thinks the equal liberty principle will be selected by rational agents reasoning from the original position because no rational agent in this position would choose to be less free than necessary nor grant some (possibly someone else) greater liberty than others (possibly herself).
The Equal Liberty Principle is only concerned with equality of liberty. But we can be equal or unequal in many other ways. In fact, being equally free is liable to lead to other sorts of inequalities. If we are all free to plant apple trees as we see fit, we will probably wind up with an unequal distribution of apples simply because some of us will plant more trees and do a better job of tending them. So long as this is merely the result of people exercising their equal liberties, there is nothing unfair about this. If I’d wanted more apples, I could have spent more time growing apple trees and less time playing chess.