Deregulation refers to a state-wide change that gives electricity consumers the right to choose among different energy providers.
You didn't always have a choice. Based on where you live in Illinois, you paid your electric bill to either Ameren or ComEd. By restructuring the Illinois electricity market, you now have the opportunity to pay for your electricity from a supplier other than your old, regulated utility. (Like Fighting Illini Energy.)
Why would you want to pick a supplier other than your old utility company? Deregulation opens up the electricity market to competition among these new suppliers.
And, changing providers is actually very easy. Old electricity monopolies were broken up into different companies - ones that actually deliver electricity from power plants to homes and businesses, and others that bill you for the actual electricity you use.