Cycling a tank is when you add ammonia to a tank (sometimes with fish in it, but usually fish less) for about four to six weeks to allow beneficial bacteria to grow. These bacteria will transform harmful ammonia and nitrites to less harmful substances so that when you add fish to the water, they won't die.
There's a lot of debate currently about what products like QuickStart do. Quickstart itself says that it adds beneficial bacteria to your tank and makes it automatically cycled. However, many people believe that the bacteria in products like these are not meant to be aquatic and thus die within a day or so and mess up the natural production of bacteria in your tank. There are other products that actually do have bacterial spores in them, but they won't immediately cycle your tank probably.