Yep, got that taIt's a bit more than that. Without negative feedback (connecting the output to the inverting input in some way) the op-amp has an infinite gain (ideally, in practice it's less than that, typically around 120 dB or 1000000:1) so the smallest difference between the inputs would make the output go directly to one of the rails, that's basically a comparator, all or nothing.
Bingo! That makes sense.Then, since you've added a resistor to add negative feedback you need to add another one on the input to keep the gain to 1:1 (or whatever gain you want). But then it's asymetric (the typical inverting or non-inverting amplifier configuration) so if you want a differential amplifier you need to have the same resistors on the other input to keep things balanced and also have a way to add a reference voltage (typically GND).
Thanks
It's an inexact oversimplification but that should help you understand
The image you posted is more about the fact IRL op-amps aren't ideal ones and don't like having their output directly connected to the input without some kind of resistance inbetween. It's advanced stuff that is nice to know but you can skip that for now if you want.