Hey there! Hello!
In an instance of Microsoft Excel 2016, the formula =G15 is referring to the column and row of the specific cell, respectively. An example of this would be A1, which would be the first cell in the A column, and so on. So when you put an equal sign in front, that basically tells Excel that you want that cell to contain the same information as cell G15.
I have attached some pictures of this that might be of assistance. The first shows me entering =G15 into cell G14, and the second shows the result, which is the same as G15.
Hope this helped you out! Feel free to ask me any additional questions if you have any. :-)