It means (most likely) that somebody has set a data validation rule for a cell that will only accept an entry that is a number, a number written out as 1, 2, 3 4.5 or some such.
Such a rule would be set up to protect the data integrity of a workbook or spreadsheet. It can be undone, unless the creator of the sheet has also used some password protection of the sheet.
So the prudent thing to do would be to check with the source of the spreadsheet. The imprudent thing to do would be to open the data validation dialog box (after putting your cursor in the cell where the invalid entry activates that error message) and deactivate the rule.