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.
Get started in Excel using templates: https://community.windows.com/en-us/videos/get-started-in-excel-using-templates/GB-FZXx3p9k
Learn more at the Excel Help Center: https://support.office.com/excel