Rocket.Chat discussions allow for you to have a separate conversation of larger topics in a team or channel.
Discussions are identified by the discussion symbol:
On the Navigation menu, discussions can be seen under the heading Discussions and every discussion is marked with the symbol above:
Use discussions to tackle down all specific topics related conversations.
Say for example you have a team named Engineering-Team, and you created a channel under the engineering team named Website-Development-Channel. It makes sense to break down conversation about each feature of the site to discussions for example Website-Responsiveness-Discussion where all the talk about the website's responsiveness is done.
Another use case for creating discussions directly in a team for example is: Say we need to talk about arrangements for the engineering teams meeting, we can create an Engineering-Meeting-Preparation discussion tied directly to the engineering team.