From help file: (I think it is quite simle)
In R1C1 reference style, Microsoft Excel indicates the location of a cell with an "R" followed by a row number and a "C" followed by a column number. For example, the absolute cell reference R1C1 is equivalent to the absolute reference $A$1 in A1 reference style. If the active cell is A1, the relative cell reference RC refers to the cell one row down and one column to the right, or B2.
The following are examples of references in R1C1 style.
R[-2]C - A relative reference to the cell two rows up and in the same column
RC - A relative reference to the cell two rows down and two columns to the right
R2C2 - An absolute reference to the cell in the second row and in the second column
R[-1] - A relative reference to the entire row above the active cell
R - An absolute reference to the current row