What is it?
A supply cap is a governance-set ceiling on how much of an asset the pool will accept. Once the cap is reached, new supply can be blocked even if people want to deposit.
Why does it matter?
A reserve near its cap cannot easily attract more cash. That matters if utilization is already high and you hoped new supply would cool rates.
Example
A listed LST has a 50,000 token supply cap and 48,000 already supplied. Only a thin slice of new deposits can enter until governance raises the cap.
Where can I find it?
Asset detail Reserve configuration, Supply cap. If Ronnie shows n/a, the live snapshot did not expose a numeric cap for that reserve.