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.