Leidos Flight Service has a website that allows you to view current and forecast weather and file flight plans. One of the things that I recently noticed is that it has detailed legends for the three most useful charts. The legends are useful for understand the charts and also for taking Knowledge Tests.
Radar Summary Chart – Detail
TVS = Tornado Vortex Signature. The radar/algorithms think there is a tornado, or a forming tornado, there. “VFR flight not recommended.”
Meso = Mesocyclone. From Wikipedia – A mesocyclone is a vortex of air within a convective storm. It is air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a severe thunderstorm.