Hi Alex,
That's a great question! Sitting on the peaks of the sit bones means that we're not letting the pelvis tilt (whether that's tilting forwards or backwards), we want it to be neutral. The pelvis should be stacked right under the ribcage. This helps keep your spine neutral: small inward cure in the lower back,
small outward curve in the middle back.
A trick to finding neutral pelvis is to tuck and untuck the pelvis into an anterior tilt a few times and then find the spot somewhere in between.
Or, if you're sitting, try to grow really tall, exaggerating it. Chances are, that makes you stick out your chest and anteriorly tilt the pelvis, which means you're sitting on the fronts of the peaks of the sit bones. Keep the length and reach of your spine but gently rock back to the peaks of the sit bones so your lower back doesn't over arch.
I hope that helps! :)