U1 is configured as an inverting amplifier - with another twist. As in the regular inverting amp, U1 aims to keep V- = V+ = 0 V. So, applying Vin causes current Iin = C*dVin/dt. The opamp output responds by dropping negative and drawing Iin away, maintaining the balance. But here, the current through Ci is proportional to dVin/dt: the slope. So, applying a constant input voltage causes Iin = 0, but a time-varying Vin causes Iin, which is balanced by Vout as in the regular inverting case. This circuit is used in edge detectors, rate sensint, transient detection, and controls circuits.
