I do understand.
N1 is the primary of the transformer. It is where the power enters the transformer.
N2 is a small secondary which powers IC1.
N3 is the secondary where power is taken from the transformer.
D3, D4 take the AC from the transformer and make DC.
When T1 is on, power is stored on the transformer. No power is output at this time.
When T1 is off (for the first small part of time) the voltage on T1 is very positive, it goes above the input voltage. At this time energy goes through diodes to the secondary supplies.
The next period of time noting happens. The transformer rings slowly. This is the dead time.