Complex sentence: Its drawbacks are that it is not self-clocking and it has a significant DC component component, which can be halved by using return-to-zero, where the signal returns to zero in the middle of the bit period.
Split into simpler sentences: Its drawbacks are that it is not self-clocking and it has a significant DC component. The DC can be reduced by one-half by using return-to-zero, where the signal returns to zero in the middle of the bit period.