What is over-feeding?

What exactly is over-feeding? How does one know they are over-feeding their 27-day old infant? Without searching for it on Google, I would like to know why my husband and I suspect that our baby is still hungry, 2 hours after his last feeding time during which he drank every last drop of his milk. Are we under-feeding him? This is the first time this happens so no cause for alarm. However, I’m curious as to what this could mean, especially since our baby is fed the recommended amount for a newborn (almost 1 month old) baby. We just fed him an additional ounce to keep him calm until the next feeding scheduled time. Again, he drank every last drop.