A Stroker kit, "Strokes" the engine. Depending on what you are doing to it, it will come with different componenets. Usually though, it's over sized pistons "0.030, 0.060, etc" this makes the diameter of each of your pistons that many fractions of an inch larger. you also get a crank and con-rods that are different length than what you have now. This will either lengthen or shorten your stroke, (adding or diminishing displacement). This is how you can take a 281 and make a 300 or 318 out of it.
here's the link to SHM Stroker Kits for 96-01 4.6 Engines
http://www.seanhylandmotorsport.com/stroker.html