See the Mattermost Amplifier for JIRA Project Administration for creating, deleting and editing notification policies.
JIRA has a complete event subsystem that fires events whenever anything happens inside the application. For example, an ISSUE_CREATED event is fired whenever an issue is created.
To notify Mattermost from JIRA when an issue is created, your JIRA Project Administrator will set up the correct policy. Then when an issue is created from JIRA for that project, the designated Mattermost channel will see the following: