Quantity thresholds on products allow you to disable products when they hit a certain quantity.
This is useful when you're integrating with a POS system. Dispense automatically syncs with POS systems every 10 minutes. While this is typically short enough to keep products between Dispense and your POS in sync it's possible that products can oversell because of the lag time between syncing.
Setting a quantity threshold can help alleviate issues with overselling.
To Set Quantity Threshold
Navigate to Menu > Products > Product Details > Pricing tab
Enter in a number in the 'Quantity Threshold' field
Typically a low number like 4 or 5 is sufficient, but you may need to experiment a bit to find the right number.