Amazon PPC glossary
Budget capping
Budget capping happens when a campaign spends its whole daily budget before the day is over, so the ads stop showing. It is a sign that a campaign is doing well enough to deserve a look at its budget, or that spend is going to the wrong place.
Example
Your $20 budget is gone by noon every day. Either the campaign earns enough to deserve a higher budget, or a few expensive keywords are using it all up.