Without seeing the circuit it's impossible to be specific but here goes.
By installing a capacitor across the secondary winding, which is inductive, there is an LC combination which has a natural resonant frequency dependent upon the values of C and L. In this case that is 220kHz (which seems rather high - it being in the radio frequency band)
The oscillator drive component (transistor) requires feedback in order to maintain a stable oscillation. Unlike an amplifier, which uses negative feedback to maintain stability free from oscillation, an oscillator requires positive feedback to initiate oscillation.
It is normal to take the output signal and feed it back, in the correct phase, to the active device. However, because the output is several hundred volts, it would be much too high for the transistor base drive. An attenuator is required to reduce this signal down to a few hundred millivolts. Normally, one might use a resistor divider but this is inefficient in an oscillator circuit so in this case they have made a voltage divider by arranging the C capacitor from a combination of 2 capacitors in series and tapped the feedback from the smaller one.
Capacitors in series are calculated using a formula similar to that used for resistors in parallel viz C= (c1xc2)/(c1+c2)
Trust this helps
jack