Where the reaction happens
When a solid reacts with a liquid or with a gas, the reaction takes place only on the surface of the solid.
Why that is
Because the reactant particles can only meet at the surface of the solid. A particle in the middle of the solid never encounters an acid molecule — all around it are more particles of the solid itself.
So the substance inside the solid only takes part in the reaction once the layers above it have reacted and been carried away. The reaction eats inwards from the outside.
Which means
If we increase the surface offered to the reaction, we increase the number of particles that can react at once. Breaking a solid into small pieces does exactly that — on the next page we work out by how much.