Annotate a worksheet by using comments
You can add comments to cells. When a cell has a comment, an indicator appears in the corner of the cell. When you hover your cursor over the cell, the comment appears.
Insert comments
1. Right-click the cell and then click Insert Comment.
Insert Comment
2. Type your comment. 
Comment
3. If you need to delete comment, right-click the cell and then click Delete Comment.
Delete Comment