When you place a block of wood in a pail of water, the block displaces some of
the water, and the water level goes up. If you could weigh the water that the
wood displaces, you would find that its weight equals the weight of the wood.
This doesn't mean that if you had a few blocks of wood that were exactly the
same size and shape, they would each displace the same amount of water. A block
of wood made of oak, for example, sits deeper in the water (and therefore
displaces more of the water) than does a block of pine. The reason is that it's
heavier for its size, or denser—in this case, the molecules that make it up
are more closely packed together than the molecules that make up the pine.
If you could somehow keep increasing the density of the block, it would sink
lower and lower into the water. When its density increased enough to displace
an amount of water whose weight was equal to the weight of the block, it would,
in a sense, become weightless in the water.
Making the block just slightly denser would cause it to sink to the bottom.