A “Global Field” is one that the whole table uses as if it was present in every record.
Like when you define a cell with a “Name” in Excel.
The benefit is that you change it in one place and it affects every record.
Imagine the exchange rate (Dollars-Euros) affecting the price of something… or the weather forecast affecting something else. A workaround: I can create a base with the Global Field and a Formula that equals that field in a different table. The problem is you need to link to the Global Field and then bring the info with a lookup… This may require a separate table for each Global Field which is far from ideal…