Every radio signal, Wi-Fi included, is built from sine waves. Change the amplitude, frequency, wavelength and phase to see what each one means, then add a second wave to see how two waves add up or cancel out.
A second wave with the same frequency, shifted by a phase. Where the two waves line up they add together (constructive); where one is up while the other is down they cancel (destructive). Radio waves meet like this whenever a signal arrives by two paths, such as a direct path and a reflection.
Each wave as an arrow: its length is the amplitude and its angle is the phase. Put the arrows tip to tail and the black arrow is the sum.
How tall the wave is: the peak strength of the signal. Power goes with the square of the amplitude, so halving the amplitude leaves a quarter of the power, which is 6 dB less.
Frequency is how many complete cycles pass a point each second. Wavelength is the distance from one crest to the next. Radio waves all travel at the speed of light, so the two are locked together: λ = c ÷ f. Double the frequency and the wavelength halves. That's why a 2.4 GHz wave is about 12.5 cm long and a 5 GHz wave only about 6 cm.
Where a wave is in its cycle, measured in degrees: 360° is one full cycle. Phase only means something when you compare two waves. A wave that has travelled one extra half-wavelength arrives 180° out of phase; one extra whole wavelength brings it back to 0°.
Multipath: indoors, the signal reaches the receiver directly and by reflections off walls, floors and furniture. Each copy arrives with a different phase. Moving a phone a few centimetres (a fraction of a wavelength) can turn a strong spot into a weak one.
Beamforming: an access point with several antennas adjusts the phase of each one so the copies arrive in phase at the client, adding up. See the Beamforming Simulator.
Modulation: Wi-Fi sends data by changing the amplitude and phase of its waves. See the Wi-Fi Modulation Simulator.
Real Wi-Fi waves cycle billions of times a second, far too fast to see. The animation is slowed down by the factor shown on the first chart, but keeps the right proportions: a higher frequency still cycles faster, and every wave travels at the same speed.