The metric PS Invocations represents the number of pixel shader invocations. The pixel shader is invoked once per pixel.
If you render a quad with 8x8 pixels size that is located entirely within the viewing frustum, the Pixel Shader Invocation Count is 64.
Usually PS Invocations workloads are one of the most expensive in the rendering pipeline due to the processing time required within the pixel shader. Therefore, keeping the number of invocations as low as possible will likely improve your rendering performance.
For Intel® microarchitecture code name Ivy Bridge and Bay Trail, this metric includes pixels rejected by Early-Depth test, even though the pixel shader was not actually invoked for these pixels.