A CAD flow is a software that allows steps in the design process to be executed automatically and repeatedly with minimal user interaction. A CAD flow can be as simple as a shell script or a makefile that will compile, elaborate and simulate a design every time you update the Verilog file. Or as complex as a regression environment that selectively runs only necessary tests when certain design components/IPs are modified and do it nightly on the compute farms, and if compute is not available, it will run on the public cloud, and switching to different simulators if the license is not available, and etc., etc.