These are all ways of measuring sugar content in grapes and must.
They work with different scales and are used in different countries, but all ultimately relate to the potential alcohol content of the wine.
A very simple tool called a refractometer is used to measure this by simply placing a drop of the juice from a grape on a small plate and then measuring the sugar concentration by the refraction of light.
We check the sugar concentration to decide on harvest time along with checking the phenolic ripeness of the grapes.
The sugar in the grape must is fermented into alcohol and each gram of sugar that’s fermented will turn into about a 0.5 gram of alcohol.