Content Construction Kits (CCKs) are extensions that allow you to build and re-arrange content using custom fields, etc.
Let's take a standard Joomla Article for example. It has fields for Article Title, Published Date, Author, etc. But what happens if you want to add fields for Editor and Publisher so that you can give credit those people who edited and published the Article? It's not possible using the Joomla core, however if you had a CCK then you could add those fields using the custom fields options. You could also rearrange the fields so that they display in the order that you'd like them.
So CCKs are ways to add additional flexible functionality to content.