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Reviewed tutorial "Explore Working Applications of 8 Different Op-Amp Circuits" (https://www.youtube.com/watch?v=aR9ILhbG4AE&t=317s)
@12:14 he indicates turning on the Reference Voltage which drives the opAmps input. Is this Reference Voltage a voltage divider connected to Non-Inverting input (pin 3)?
ronyd Derek is doing a demonstration of a summing opamp. He put DC on one of the input resistors going to the inverting input and he adds a sine wave - the reference voltage he mentions in the clip - to the other resistor connected to the inverting input. The non-inverting input is connected to ground. This gives him an AC waveform on the output with a DC offset.
I understand the confusion because he does mix the inputs up (once) while he's describing the circuit. 3 (+) is the non-inverting input shown in the schematic a few moments before in the video. Doing instructional videos is much harder that many folks realize. Viewers expect a perfection standard and that's just not human. Besides, there is a benefit to catching the little "oops" along the way. This is how we improve our troubleshooting skills.
Hope this helps.
* I editted this post TWICE already. "Perfect" is an unrealistic expectation. 
