Yes, that's R1C1 notation.
If you prefer you can convert:
Sheet1.Range("B1").FormulaR1C1 = "=LEFT(RC[-1],3)"
Sheet1.Range("B1").Formula = "=LEFT(A1,3)"
You may find it useful, especially with more complex formulae, to become familiar with the ConvertFormula Method. This will do the conversion for you.
For example, in the Immediate Window enter:
debug.print Application.ConvertFormula( formula:="=LEFT(RC[-1],3)", _
and then press <Return>
You will be presented with : =LEFT(A1,3)