Three conditions can trigger this warning to show up in showplan:
Excessive Grant: when max used memory is too small compared to the granted memory. This scenario can cause blocking and less efficient usage when large grants exist and a fraction of that memory was used.
Used More Than Granted: when the max used memory exceeds the granted memory. This scenario can cause OOM conditions on the server.
Grant Increase: when the dynamic grant starts to increase too much, based on the ratio between the max used memory and initial request memory. This scenario can cause server instability and unpredictable workload performance.
I like that this information also shows up when you view an execution plan using SSMS 2014 SP2.