Pitch rate is the number of healthy, viable yeast cells added relative to a batch's volume and gravity — usually expressed as cells per millilitre per degree Plato, a way of normalizing pitch size against both how much wort there is and how much sugar it carries.
Two failure modes, not just one
Underpitching (too few cells for the batch) stresses the yeast population that is present, forcing it to reproduce more than it otherwise would and often producing more fusel alcohol and off-flavor character as a result, alongside a real risk of stalled or incomplete fermentation. Overpitching (too many cells) is less commonly discussed but genuinely real too — an oversized pitch can ferment unusually fast, strip out desirable ester character a style depends on, and sometimes finish drier than intended because there's simply more yeast working through the available sugar than the recipe assumed.
Why gravity changes the target, not just batch size
A yeast starter or a rehydrated dry yeast packet correctly sized for a standard-strength ale is genuinely undersized for the same batch volume at Barleywine or Imperial Stout gravity — pitch rate calculations scale with both volume and gravity together, which is why every high-gravity style page on this site repeats the same advice about a larger, healthier starter rather than treating pitch rate as a fixed, one-size-fits-all number.