Relative reference copying
You are copying cell C2 to cell C3 (i.e., select C2, press Ctrl+C, then select C3, and press Ctrl+V). Cell C2 has a formula =A2+B2. After copying, what will the formula in cell C3 become?
=A3+B3=$A$2+$B$2=A2+B2=$A2+$B2