I've just got an Excel file one day when I was working really hard, the problem was that this file contained a lot of numbers with no labels, which I considered something really strange, but suddenly I realized that the description of each calculation was as a comment, so I decided to write about it.
As you know very well, Microsoft Excel allows you to put comments on every cell, what it's very useful at the end when you want to explain something due to the fact that Excel it's not for writing long text. To do so, just go to the ribbon called "Review" and click on "New Comment" and then you can write anything you want in the tiny yellow space that appears.
Well, that's ok but I don't want to be clicking all time in the same button to get a comment inmmediately. Yeah, you're right, this is why I have a little trick, just press "Shift" + "F2" keys to create a comment in one shot and also you can use it to edit any comment you find in your file, in this way you will save you some time and effort.