All transformers have leakage inductance, which essentially means uncoupled inductance.
This can be modeled as a series inductor. With the series capacitor it will form a high-Q LC tank that will resonate.
Depending on the actual switching frequency, and the actual LC values, this resonance could become excessive.
The series resistor will help dampen that resonance.
But its value must remain low, otherwise you will limit the actual drive current.
The best value is often based on empirical measurements.