In a crosstab, the column headings (i.e. description of what's in the column) is determined by your data...for example, if your columns are based on Year then the different values of Year determine the headings. It wouldn't make sense to have every column just say "Year".
However, if you want to add text item to your header, you can unlock the crosstab and drag a text item into your Crosstab Level cell. Like any text item you can then put in static text or point it to a query item (for a query item you'll probably want to set its content property to Label). You'll probably also want to set its box type to Block to force wrapping, so the text item appears on a line by itself above the crosstab level value.
