Marketplace apps
Jira has lot of features out of the box and it comes with a few templates based on the type of application you use. For instance, when using Jira Software, you can create a project with either Scrum- or Kanban-based configurations. At the same time, you always have this option to modify and create your own set of configurations using various schemes in Jira. However, apart from the standard features in the tool and these customizations, if you need to extend the feature set of Jira, you can install various plugins better known as add-ons or more recently as apps.
These apps are either from Atlassian itself or from other companies—mostly Atlassian partners that have published these apps on the Marketplace:
https://marketplace.atlassian.com/
If you open this link, you'll be taken to the Atlassian Marketplace place where you can download thousands of apps for not only Jira but for all other Atlassian tools. As displayed in the following screenshot, you can either browse various apps or search for them:
When you go to the Marketplace, you have the option to look at various popular apps for each Jira application but let's say you want to do Test management in Jira and you're wondering whether an app can quickly give you this option; you can download various apps from the Marketplace and evaluate them.
Having these apps gives you endless possibilities in Jira—of course, it depends on whether the app is good enough for your use case or not, but at least you know that, with the help of an app, you can do what you can't do out of the box. These apps either provide more functionality or provide integration with other tools.