Why is it that when mRNA of nematode is silenced , the parasite dies? What silencing actually means?

When mRNA of nematode is silenced , the parasite dies because it is unable to parasitise  the host cells. The parasite becomes unable to obtain nutrition from the host cells.  

Nematode-specific genes  which are introduced into the  plants  to develop resistance in  plants against nematodes are nematode parasitism genes. These are those genes which help the nematode to successfully  parasitise the plants. 

When this plant is infested by the nematode, the nematode specific gene is triggered and it produces a single stranded RNA which is complementary to the RNA produced by the nematode in order to infect the host plant. This complementary RNA binds to the mRNA of the nematode to form a double stranded RNA that cannot be translated. Hence, the nematode is not able to infect the trangenic plant.

 Silencing of mRNA means not allowing mRNA to get translated into a functional product due to formation of double stranded RNA.