Teams - Apps and Tabs
You can add relevant files and Office 365 tools/apps to tabs at the top of any chat or channel; you can also add Office 365 tools/apps to meetings to help you collaborate better with attendees. Note: All third-party apps are disabled by default.
Apps can show up in several places, depending on what they do and how they've been built.
Some apps are built into Teams, like OneNote.
Apps that work with messages let you insert their content directly into a message.
Other apps can send notifications and information to a channel. These are called connectors and they connect to things like Twitter or Yammer, or your project management software.
Some apps can be used outside of a Team, channel, or chat. When you access an app in this way, you get a view of all of your information in that app from across Teams.
Teams - First things to know about apps in Microsoft Teams
Teams - Video: How to use apps in Microsoft Teams