The transistor is biased in class A which means that collector current flows all
the time. The collector current can increase or decrease. The input signal increases and decreases the forward bias causing the collector current to change.
These changes in current in the primary of the transformer induce signal currents in the secondary. The transformer matches the output impedance of the transistor to the loudspeaker impedance.
The disadvantage of this circuit is that the collector current is high even if there is no signal input.
Class B push-pull is more efficient.