The document discusses wave energy and interference. It defines standing waves as occurring when a traveling wave is reflected by a fixed boundary, resulting in the superposition of the original wave and reflected wave. Standing waves have nodes where the displacement is always zero, and antinodes where the displacement is at a maximum. The normal modes of a system are its allowed standing wave patterns, which are determined by the boundary conditions. For a string fixed at both ends, the normal modes are half-wavelengths that are integer multiples of the string length.