This is extremely basic, but assuming it is a serious question, the 'swim' is the shape of the hull at the stern as it comes to a more or less pointed shape where the propellor is. The finer this shape is the easier it is for the water to be drawn by the propellor and the less 'drag' created. This means the boat has less wash, less turbulence, greater efficiency and greater speed. The counter is the deck platform above this where a steerer stands on an ex-working boat or anything based on that hull shape.
It is much cheaper and easier to build a straight-sided hull until the last moment and come to an abrupt pointy bit for the propellor, but that gives you a far less manoeuvrable craft. At the extreme you can have something that is simply a box shape with no swim at all - simply a propellor poked through the back of the box. You get more internal space but the boat is virtually unsteerable. Working boats have a fairly long and fine swim as ease of steering, fuel economy etc was of prime importance. Most people buying their first boat now only look at the internal space and price, but never do understand why boating is then such hard work for them.
Tam