In Zoho CRM, data is categorized into groups or divisions based on similarity. Each division that has similar data is called a "module". For example, 'Leads' is a module that contains all your leads' details. Contacts, Accounts, Deals, Quotes etc. are other examples of modules.
In Zoho CRM, modules are of two major kinds - default modules and custom (or user-generated) modules.
All these tabs (red underlined in the screen grab above) that you see in your Zoho CRM top header, are different modules. If you click on the ellipsis (three-dots), you can see more modules.
Sometimes, a particular module can be named differently for different Zoho CRM users, based on what edition or release of Zoho CRM they are on. Here's a quick discussion on the Deals' module, that is also sometimes called Potentials or Opportunities.
Next 'Word of the Day' - Leads