I assume the question is about formula in SALARY column of Table tAnnex.

tAnnex in formula returns datarange of Table tAnnex, i.e. a range A2 : D5 in example file. NB! This range is 2-dimensional, i.e. in general it contains rows and columns.
For 2-dimensional ranges/array:
INDEX(DataRange,RowNo,ColumnNo), returns a value from single cell;
INDEX(RowNo,), returns a single row from datarange;
INDEX(,RowNo), returns a single column from datarange;

As follows, 'INDEX(tAnnex,,[@[SALARY STEP]]+1)' returns a datarange column ([@[SALARY STEP]]+1 from tAnnex, i.e. when SALARY STEP in tSalaries is 1, the column Step1 of tAnnex is returned, when SALARY STEP is 2, the column Step2 is returned, etc.