Assembly lines enable firms to mass produce products swiftly and more efficiently. Kitting involves bundling two or more similar items to create one ready-to-ship kit. Those bundles are usually sold as a single item. Also, unlike pre-assembled products, the end consumer usually assembles the items in kits by themselves.
Be prepared that even if those two words have slightly different meanings in manufacturing, some people still prefer using kitting and assembly interchangeably.