Numbers error : What does this mean? "This formula can’t reference its own cell, or depend on another formula that references this cell."

You are wanting to add up the contents of a number of cells, so if you are adding A1 to A8 then cell a9 might contain the formula '=SUM A1 + A2 +A3' and so on up to A8. This will work. However if your forumula has ended with '...+A9' then since A9 is the result of a calculation it value can only be determined as the result of the formula, and so when it's included within the formula its value cannot be known.

Since a more usual formula would be '=SUM A1:A9' (or if more than one column is involved, something like '=SUM A1:C9' it would be easier to miss an inadvertent inclusion of a cell with a forumula.

Do you understand what a formula in a Cell is? How do you add say your expenses and display a total in another cell?

If you are manually adding the totals, then this would not be applicable. but if you use a formula to produce a result from adding other cells together for instance, this formula cannot reference its own cell where the result is displayed.

Cells can contain formulas to provide calculations. Like adding all the expenses and displaying the total in a cell at the bottom of the list of expenses labeled Total.

The sum of all the expenses, cannot include the cell where the total is displayed, or include a cell that uses that total for a different calculation.

If you have 5 expenses in 5 cells, and then you add them all up in the bottom cel using a formula, =SUM a1:a5 in the cell for the total, the cells labeled as a1, a2, a3, a4 and a5 cannot themselves include another calculation that uses the cell displaying the total to get a result.

If this still does not make sense, then you are probably not using formulas, and you just mistyped something in a cell, and Numbers assumed it was a formula.

It means exactly what is says. No gibberish. It's pretty clear.

The formula in the cell in question is either referencing the same cell or referencing another cell that in turn references the first cell, so it cannot be calculated.

For example if cell1 contains cell1+cell2+cell3, it references itself, so there's no way to calculate a value, because there's is no way to get a value for cell1, if its recursively using its own formula for its own value calculation.

Or if cell1 contains cell2+cell3, but cell2 contains cell1+cell5 for example, then again, it cannot calculate the value because it goes back to the original cell that is trying to calculate a value which it doesn't have for the other calculation.

Formulas need to have knowable values. A cell with a formula that references itself can never get a value, because there's no way to calculate it if its trying to use same value it's trying to calculate.