They are Excel error messages.
#NULL! normally means you made an error in specifying two sets of ranges in a formula.
#NUM! can mean a couple of things. First, it can indicate that the result of the formula is too large to be represented in the spreadsheet. Second, #NUM! can indicate that you provided an invalid argument in a formula. For instance, Excel will display #NUM! if you attempt to take the square root of a negative number.
#N/A means no value can be computed for a formula. For instance, Excel will display #N/A if you attempt to compute the mode of a set of data that doesn't have any duplicates.
#REF! means you deleted a range of cells that were originally used in a formula. For instance, if cell A1 contains the formula =AVERAGE(A10:C10), then you delete the tenth row of your spreadsheet, Excel will display #REF! in cell A1 to tell you that the formula's references have been lost.
#VALUE! means you entered a mathematical formula that references a text cell. For instance, Excel will display #VALUE! if you attempt to take the square root of "John Mote."