Diffraction is when waves bend around the corner of an obstacle.
It is most easily seen when a wave spreads out after passing through a gap.
Medium gap: some diffraction, but mostly straight
Wavelength-size gap: most diffraction
The maximum effect is when the gap and wavelength are similar in size.
Here we see diffraction as waves enter a harbor:
Diffraction is very important for Radio Waves: it lets them get around hills and into valleys:
Radio waves with wavelengths of kilometers diffract
over hills and through valleys so you can easily get reception.
But microwaves with wavelength of centimeters tend to go straight.