Inverting gain
Think of a seesaw pivoting at the (−) input: Rin and Rf are the two arms, and the output swings the opposite way by their ratio.
|A| = Rf / Rin (output is inverted)
Ideal inverting amplifier: the output is flipped in sign and the (−) input sits at virtual ground. Rin loads the source. Gain says nothing about bandwidth, output swing or offset; check the op-amp datasheet and supply rails.