Cookbook is the basic building block of chef configuration management tool. A cookbook contains the complete set of resources required to configure an application or certain thing on a node(a node is a server that has chef agent running, which will pull configurations applicable from chef server).
For example, a cookbook for Apache or Nginx web server will have all the installation, configuration options to completely get it running on the node.
If you have previously worked with Puppet configuration management tool, then you can compare chef cookbooks to puppet modules.
Before proceeding further with this article, reading the below articles that gets you started with chef configuration management tool is necessary.