This circuit inverts and amplifies the input, multiplying the voltage by -3, using an op-amp. When connected in a negative feedback configuration, the op-amp attempts to keep its two inputs at the same voltage. One is at ground, so for the other one to be at ground, there must be a voltage drop across the 1k resistor equal to the input voltage. The 3k resistor has the same current across it, so the voltage drop must be 3 times as large, by Ohm's Law, making the output equal to -3 times the input voltage.
Next: Noninverting Amplifier
Previous: Op-Amp Feedback