Vector network analyzers (VNAs) measure how a “network” of components changes the amplitude and phase of signals. By measuring across a wide frequency range, VNAs can create S-Parameters that fully describe the behavior of a circuit. This video uses a PicoVNA 106 to show the basics of what a VNA measures, how they work, and a brief overview of how to use one.
I made a mistake on the explainer graphic for S-parameters. The annoying thing is that I think it was a really good explanation!