Selected Answer
The cell L2 has "Data Validation", meaning there are restrictions as to what you can enter in that cell. The "Comment" you see is the "Input Message" associated with the validation setup. To set, remove or modify the Data Validation click the Data Validation button in the Data Tools meno of the Ribbon's Data tab and select Data Validation to open the related dialog box.
You can edit the text of the message on the Input Message tab of that dialog. You can give it a title which will appear bold in the message, or you can delete the message in which case there will be none although the validation will still be active. Actually, the message indicates that a date should be required and Validation should reject when a non-date is entered. But on the Settings tab you can see that Any value is selected and that means that there are no restrictions. You are right: the iinput message serves no purpose. If that is what you want, delete the entire Data Validation which will also do away with the message. There is a Clear All button on the Settings tab for this purpose.