What is the de Broglie wavelength? Why do we care?

The de Broglie wavelength is an important idea in physics that shows that all matter with mass can also show wave-like properties. For example, an electron (which we usually think of as a particle) can diffract around a corner. Heavier objects like molecules can also do this in extreme circumstances. Even something as large as a tennis ball could theoretically diffract. However, we can show using the equation (lambda=h/p) that this is impossible in practice.-> At this point I would show why a tennisball doesn't diffract and an electron does diffract a lot more easily, using the equation. -> Then I would ask the student to explain it back to me so I can see how much they understood.