In Excel, the little red triangle in the top right corner means there is a comment associated with the cell. To see the comment, you simply roll your mouse over the triangle. It should show the comment in a pop-up box. From there, you can read the comment and make adjustments as necessary, or take something into consideration. This will allow you to do as you see fit.
Making a comment in Excel is just as easy, for those who don’t know yet. Just as you would in Word, you go to the “review” tab. Then, you click “new comment”. This will bring up the comment box that you type in. The commenter’s name will appear in the box, bolded. All you do from there is type your comment, and click out of the box when you’re done.