A refinery may have plenty of naptha and might want to break it down into smaller more valuable molecules. This is where cracking comes in. When any alkane is cracked you always get smaller alkanes plus some alkenes. Lots of ethene molecules can, under certain conditions, join together to form a polymer called polyethene. This molecule is called propene, notice that it has three carbons in the chain.
Propene polymerises to give polypropene.
Polypropene can be moulded or spun into strong fibres.