In Microsoft Excel, comments are used to provide additional information or explanations about specific cells. They serve as annotations or notes that can help document calculations, provide context, or communicate important details within a spreadsheet. Comments can be added to cells to provide clarity and make it easier for others to understand the data and calculations in the spreadsheet. They appear as small indicators in cells, and hovering over the cell reveals the full comment text. Comments are useful for documentation, auditing, and facilitating communication within Excel spreadsheets.