Where waves meet.

When waves overlap, their displacements add. Equal in-phase waves reinforce; equal opposite displacements cancel momentarily. Reflections can create standing waves with fixed nodes and antinodes. Adjacent nodes are half a wavelength apart.

For a string fixed at both ends, allowed wavelengths satisfy L = nλ/2. Resonance produces a large response when a driving frequency approaches a natural frequency; damping limits the amplitude. Real measurements need uncertainty estimates and repeated trials before a model can be evaluated.