Why ethene is used in the production of polythene?

Ethene is an organic molecule containing two carbon atoms
linked with a double bond, with two hydrogen atoms bonded to each of the carbon atoms.  
the double bond means that there are a total of 4 electrons shared between the  two
carbon atoms.

Ethene is also what is known as a monomer.
This is like a single link in a chain.  In a polymerization reaction, one of the bonds
between the two carbons opens up and joins with another, separate ethene molecule. 
These  reactions happen very rapidly and ultimately you can have hundreds, thousands, or
even millions of ethene molecules joined together, just like a long chain necklace which
contains many individual links.

When ethene becomes a long
chain polymer its name changes from ethene to polythene.  Poly meaning many, and the
thene from the name of the ethene monomer which started the
chain.

Essentially all polymers are named by combining the
word poly with part or all of the name of the monomer.