Tidy data:
Your data file should be a neat rectangle. No empty rows. No empty columns. Neat, clear headers on the top (only taking up one row). A common problem with "untidy" data is the use of hierarchy, where it lists something -- perhaps a state name -- and then below that are items related to that state, but nowhere on the line does it say the state name. You can't sort this kind of data and it might be difficult to do a Pivot Table or any sort of analysis.
You can see some examples of untidy data here and here. These are truly just "reports" that are stored in Excel.