SDA is “Serial Data Line” and SCL is “Serial Clock Line” of the i2c bus which is used by the display and the DAC. SDA is physical pin 3 and SCL is physical pin 5 of the 40pin GPIO connector on the Pi.
The wiring shown in the pictures on this page in section “Step 1” is wrong for newer Pis. They are only required for old Model A/B boards where the DSI connector does not have I2C connections. With your Pi 3B Plus SDA and SCL signals are transmitted via the DSI ribbon cable so the yellow and the green wire would have to be removed.
@volumio Since this wiring issue has been reappearing multiple times IMHO the wiring instructions in the tutorial should be reworked as described here.