Obtaining frequency of 550 Hz from Simple Triangle and Square Waveform Generator Circuit
I know that the second op amp acts as a comparator, right? So I'm
guessing the voltage between R3 and R2 is the threshold. But I'm still
a little lost on how that relates to the frequency of the signal.

The right hand LM324 is acting as a comparator with hysteresis and the level of that hysteresis is determined by the resistors R2 and R3. This means that when the op-amp changes state on the triangle waveform voltage hitting the upper hysteresis limit, the new level is made significantly smaller so that the triangle wave has to drop down so far before the op-amp (aka comparator) switches again.
This all means that R2 and R3 do affect the oscillation frequency - more hysteresis means a lower frequency and more hysteresis is caused by R3 lowering in value or R2 rising in value. Both affect the hysteresis and hence the frequency of oscillation.
Look at this diagram to see what I mean regards the op-amp having an upper and a lower threshold: -

Note that the output (the square wave changes when the input crosses a threshold.