Serial or Parallel resonance.
When a crystal is described as being a particular frequency series or parallel resonant, it is analogous to any other part being specified at a temperature of 20 or 25 degrees. The part is the same but the frequency has been determined under those defined parameters. At series resonance, the crystal appears as a series resonant RLC circuit with 0 phase shift, so to use it in an oscillator you need to provide the 360 degree phase shift needed. At parallel resonance, the crystal is predominantly inductive and with the external capacitors, provides 180 degrees of phase shift at the specified frequency. The other 180 degrees is provided by the inverting amplifier provided in the micro. The difference in frequency between series resonance and parallel resonance is about 0.05%