The EU Idle % metric represents the percentage of time when the GPU execution units (EUs) were idle. An EU is idle when it is neither actively executing shader instructions nor stalled trying to execute shader instructions.
If EU Idle % is 50, it means that the EUs were idle for 50% of the rendering time for selected ergs.
If EU Idle % is 0, it means that the EUs were either active or stalled for the entire duration of the rendering time for the selected erg.
If EU Idle % is significantly higher than 0%, this indicates that there are stalls elsewhere in the rendering pipeline.