When the voltage between the trigger pin 2 and ground pin 1 falls (press switch) pin 3 voltage rises and discharge pin 7 is disconnected. The capacitor charges until threshold pin 6 and ground reach 2/3 supply, pin 3 voltage falls and pin 7 reconnects. As a monostable the timer returns to output 0V after a time set by the resistor/capacitor. As an astable, pin 2 connects to pin 6 producing an automatic restart. These clock pulses can drive a flashing LED, a loudspeaker, a transistor, an IC, etc.
A 741 operational amplifier or op-amp is used as a comparator or amplifier with one output and two inputs. A voltage at the + input causes a positive output (source) if it is more than the - input and a negative output (sink) if it is less than. The voltage gain is 100 000 and the output current is 10 mA. As gain is very high resistors are used to produce negative feedback and give a lower but predictable and constant voltage gain.