The EU Active % metric represents the percentage of time when the GPU execution units (EUs) were actively executing pixel, geometry, or vertex shader instructions.
If EU Active % is 80, it means that the EUs were active 80% of the rendering time for the selected ergs.
If the EUs are not active, it means that they are either stalled waiting for a request to be fulfilled, or idle. You can see how much of the non-active time is caused by stalls by examining the EU Stall % metric. If the total EU busy time (EU Active % + EU Stall %) is significantly lower than 100%, this indicates that there are stalls elsewhere in the rendering pipeline.