Modules are the workhorse of Ansible. They provide just enough abstraction that enables playbooks to be stated simply and clearly. There are over 100 modules maintained by the core Ansible development team covering clouds, commands, databases, files, network, packaging, source control, system, utilities, web infrastructure, and so on. In addition, there are nearly 700 other modules maintained by community contributors that expand functionality in many of these categories. The real magic happens inside the module code, which takes in the arguments passed to it and works to establish the desired outcome.
Modules in Ansible are the bits of code that get transported to the remote host to be executed. They can be written in any language that the remote host can execute; however, Ansible provides some very useful shortcuts if writing the module in Python.