If you have not seen this, try putting few oranges inside a bucket of water. You might see that oranges float on water. Now if you peel the orange and again drop them into the bucket, you would see them sink.
We know light objects float and heavier objects sink. The tough outer skin of orange also called rind contains pockets of air. These air pockets make the orange less dense than water. That’s the reason it floats on water. When the rind is removed, the air pockets get removed meaning air is no longer held in the fruit and density of the fruit causes the fruit to sink.
Density of a material is its mass for a given volume. Factor affecting the density of a material is how concentrated the atoms are in a given volume. The difference in density is due to the arrangement of atoms in the rind and the fruit. The atoms in the rind containing air pockets are loosely packed compared to the atoms in the fruit without the rind. Hence the fruit with rind is less dense than water but without rind is denser than water.