You may often insert comments to the contents in Google Sheets. Please see below for details:
Step 1: Right-click the cell that you want to insert comments;
Step 2: Move down to the bottom of the dialog box and select "Comment";
Step 3: Type your comments in the box and click the "Comment" button;
Step 4: The "Comment" will appear when you hover the cursor on the cell;
Step 5: If multiple people work on the sheet, other people can reply to your comments;
Step 6: Click "Resolve" in the comment box to remove the comments from the cell;
Step 7: To check the comments after, click the "Open History" button on the top-right corner;
Step 8: You will see all the resolved comment here;
Step 9: You can delete the comments anytime by clicking "Delete" after the comment.