Shiny's module feature lets you break complicated UI and server logic into smaller, self-contained pieces. Compared to large monolithic Shiny apps, modules are easier to reuse and easier to reason about. See the article at http://shiny.rstudio.com/articles/modules.html to learn more.
callModule(module, id, ..., session = getDefaultReactiveDomain())A Shiny module server function
An ID string that corresponds with the ID used to call the module's UI function
Additional parameters to pass to module server function
Session from which to make a child scope (the default should almost always be used)
The return value, if any, from executing the module server function