Only '@' or something like '[@YourColumnHeader]' or '[@[Your Column Header]]'?

When more than '@', then you probably defined your regular Excel table as a Defined Table, leaving formulas there not converted into format used by Defined Tables. And when you now edit some formulas, or add new rows there, the Defined Tables formula syntax is applied for updated/added cells.

When you create a Defined Table, you determine for it a name (e.g. tMyTable]. Column headers in Defined Table will be names of data columns. NB! It will be a good practice having no spaces in column headers, as otherwise you need an additional pair of square brackets in every reference to values in this column.

Now, to refer the whole datarange in some column in same Table you have designing the formula, you do this like:
[YourColumnName] or [[Your Column Name]];
To refer to whole datarange in some column in some other Defined Table anywhere in your workbook, you do this like:
NameOfOtherTable[NameOfColumnInOtherTable];
To refer to cell in same row in some column in same Table you have designing the formula, you do this like:
[@YourColumnName]
(NB! You can refer in this way to cell with same row number in any other Table too, by simply perceeding the reference with source Table name, but this assumes, this other Table must be always aligned with one you enter the formula into - practically you never can guarantee this!);
To refer to whole datarange of any Defined Table in your workbook, you do this like:
NameOfTable;
To refer to certain header cell in any Defined Table, you do this like:
YourTableName[[#Headers];[YourColumnName]]
To refer to whole header range of any Defined Table, you do this like:
YourTableName[#Headers]

To refer to cell in another row as the one you are entering the formula in, there isn't any simple way. You either refer to it using regular syntax, or you do this like INDEX([YourColumnName],RowNumber). The row number you enter either manually, or you can have a column in your Table, where all Table rows are numbered by formula automatically, and you use SUMIFS() to calculate the row number which must be used.

NB! Whenever you change the name of any Defined Table on workbook, or name of column in any of Defined Tables there, all formulas in this workbook using Named Table syntax will be updated immediately with this change!