This is a subtle and slightly tricky thing to explain. When you have an object that you'd say is heavy, that means two things. One is that it has a lot of "mass," which is a way of saying that it's hard to get it to start moving from rest. This is true even in the absence of gravity. For example, if you have an object with a lot of mass out in space away from anything that pulls on it with gravity, it's still harder to get it moving than it would be to do the same with an object with less mass.
That said, if gravity pulled on all masses the same, then you'd expect that things with more mass would fall more slowly than objects with less mass. After all, gravity would pull the same on everything, but it would be harder to get the heavier things going.
The weird thing is that gravity somehow knows to pull on things with more mass more than it pulls on things with less mass. The effect is exactly such that it cancels out the fact that more massive things are harder to move. For example, suppose you drop two objects, one with a mass equivalent to one apple and the other with the mass of two apples. The one with mass of two apples is twice as hard to get moving, but gravity also pulls on it twice as hard, so it falls the same way as the less massive object would.
If you find the idea weird that the earth somehow "knows" to pull harder on massive objects than on less massive ones so that they all fall the same, you're not alone. Albert Einstein replaced this view (which was due to Isaac Newton) with a different one where gravity is not seen as a force at all, but rather a curvature of space (and time) along which everything moves the same way if dropped. This removes the mystery of how the earth knows to pull more on more massive objects so that everything falls the same way, but does strain the old brain a bit!
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.
