[linear attenuation coefficient] What does this graph mean?

1) Why does it has "peaks" on lower energy?

2) Why does start high for lower energies and low for higher energies?

3) Why u/p and u(en)/p are basically the same for lower energies but then, for higher energies they're different?

1) Why does it has "peaks" on lower energy?

2) Why does start high for lower energies and low for higher energies?

3) Why u/p and u(en)/p are basically the same for lower energies but then, for higher energies they're different?