CPU has done all the work the GPU can feed it and goes IDLE. the GPU is bottlenecking the CPU, forcing the CPU to slow down. or vice versa, the GPU is giving the CPU more information than it can process causing the GPU to throttle or drop frames or other issues. the CPU is the bottleneck.
Ideally you want the CPU and the GPU to be balanced more, there will always be a bottleneck the key is to make the difference as small as possible.
bottlenecking happens a lot in storage. putting an external SSD on a USB port that is generation 1 would bottleneck the SSD. USB 1 is slow as in comparison to what the SSD can do. USB1 is the bottleneck. plug the same drive into USB 3.0 and the wider path removes the bottleneck.