What happens when you enter the formula G15 into a cell in excel?

If you just type G15 into a cell, it is not a formula but just a piece of text. If you put an equals before it, then it will display what is in the cell G15 when you type it into any other cell.

=G15

If you type it into G15 itself you will get an error known as a circular reference, because a cell cannot mention itself in a formula.