The first range (D4:D11) are the cells to sum, called the "sum range".
Criteria are supplied in pairs...(range / criteria).
The first criteria pair is (B4:B11 / "blue"). This means cells in B4:B11 must equal "blue".
The second criteria pair is (C4:C11 / ">15"). Cells in C4:C11 must *also* be > 15.
These cells in the sum range will only be summed when all criteria pass.