As in Drupal and Wordpress, an enormous amount of functionality is provided by contributed modules (often referred to by the shorthand “contrib”).
For people upgrading to Backdrop from Drupal 7, the vast majority of the top 100 Drupal 7 modules have either been ported to Backdrop or are already present in Backdrop core.
There are over 1000 modules available for Backdrop, with more being added each week. There are several resources to find the modules you may need:
Backdrop Modules Page
The official distribution channel for Backdrop modules is the Backdrop modules page on backdropcms.org. This lists all modules that have an official release (or pre-release). Try some keywords to find modules that address particular needs. To find ports of Drupal 7 modules, use the name of the module.
Github backdrop-contrib Repository
You can find all of the above, plus modules still in development, in the Backdrop Contrib Github repository. Modules that haven’t been released yet are of course likely not ready for production usage, but if you find something that looks applicable, you can try it out, and if it looks close, drop a note to its developer expressing your interest (or possibly your willingness to help bring it to production).
If you can’t find a module that does what you want in the above, there is the option of developing it yourself—whether porting an existing Drupal 7 to Backdrop, or creating a module with new functionality ab initio.
Documentation for converting Drupal 7 modules can be found here. Before starting a conversion, you should check the Drupal 7 module project issue queue to see if a Backdrop port has been started (look for the tag “backdrop-port”); if you don’t see one (and you don’t see a port in the Github backdrop-contrib repository), then it’s probably safe to start the port.
Documentation for developing new contributed modules can be found here.
Contrib Module Documentation
Most contrib modules have documentation in the module README.md file. Some also have documentation in the Wiki page of their Github repository (which will be linked-to from the README file).
Some contrib modules are part of a larger ecosystem and have additional documentation available in the sections that follow.