Simple harmonic motion
An undamped mass oscillating on a spring produces a sine wave. Note that a maximum or minimum value of position corresponds with a zero in velocity. Acceleration is not constant, but varies out of phase with position vs. time.
A rotating object can also be considered a simple harmonic oscillator. Its projection produces a sine wave.
The phase is the argument of the sine function. The phase constant provides initial conditions for the oscillation, when t = 0.
Simple harmonic oscillator: mass on a spring