That's the schematic of a microwave oven. 3 is the magnetron tube. It's anode is tied to GND. That's one of the rare cases where GND has a positive potential against other parts of the circuit. The mounting inside the oven is much simpler that way.
Because a magnetron tube is a diode itself, the circuit needs the antiparallel diode D1. A transformer has to deliver AC, it would saturate if there was a DC component. Instead of using a rectifier bridge, and instead of throwing away the energy of one half wave, the circuit also acts as a voltage-doubler. That's the purpose of C, which is a series resonator together with the secondary main winding of Tr. The purpose of D2 is limiting the voltage over C to avoid C fail miserably (it could explode on overvoltage.)