This piece of equipment consists of a glass or plastic tube of around 5cm in width and 60cm or so long. At each end of the tube is a bung, with one end having a metal tube going through it, this is connected to a length of pressure tubing and then to a vacuum pump. Somewhere on the pressure tubing will be a Hoffman clip. Inside the tube is a guinea (usually a small coin but for safety reasons, a thick rubber washer is sometimes preferred) and a feather (or small piece of tissue paper).
In air, the guinea and the feather fall at different speeds due to air resistance. To show this, turn the tube upside down so both items fall from one end of the tube to the other. As you will see, the guinea will fall much faster than the feather because it is affected less by air resistance. (The feather has a greater surface area which comes in contact with the surrounding air).
Turn the vacuum pump on and expel as much air from the tube as is safely possible. When you have done this, tighten the Hoffman clip and turn off the pump. Inside the tube you should now have a near vacuum. Turn the tube upside down once again and you will see that the guinea and feather arrive at the other end at the same time. With no air in the tube, both objects fall at the same speed as there is no air present (therefore no or negligible resistance) to affect their descent.