In Microsoft speak, a Conversation is the complete chain of emails from the first message through all responses, and the messages of a Conversation have the same subject.
The new feature, called Ignore Conversation, is specifically aimed at getting rid of company-wide email threads that build up in the user's inbox as other people keep replying to them.
Microsoft's release notes for version 4.36.0 of the app describe Ignore Conversation as follows:
Sick of that company-wide email that everyone is still replying all to? So are we. Stay focused on what's important by getting it and all future replies out of your inbox and straight into deleted items with our new Ignore Conversation feature.
The feature has actually been available in Outlook on other platforms for some time, but this is the first time that users accessing their emails on iOS devices have been able to use it.