Answer:
the equal sign '='
Explanation:
Entering an equal sign ('=') as first character in a cell indicates to the spreadsheet program that the contents of that cell is to be treated like a formula, not like text nor number.
That way, you can write a text saying for example "A2+A3 contain this data" directly.
However, if you want to do an addition of A2 and A3, you'll enter "=A2+A3";
