Linearized phase domain model of a PLL - what does a ratio of input phase over output phase mean?
A key point not mentioned in the other answer is, when we talk about a "linearized" model, we're talking about small oscillating deviations about an operating point. 

As clabacchio said, the point of a PLL is to generate an single-tone output whose output phase is aligned to the phase of a single-tone reference input, typically at a lower frequency. The linearized model tells us, if the phase of the input signal changes by a small amount, how much will the phase of the output change. Furthermore, the rate of change of the input is important, so we represent its phase by a phasor variable, and the transfer function tells us not just by how much the output phase changes, but how its change lags or leads the change in the phase of the input signal.