Immediately after installation, the app starts indexing data about builds available on the TeamCity server you specified and maps them to the corresponding issues. Additionally, the app adds the TeamCity tab into the project and issue view.
Issue Key is a Jira issue ID. Normally, it's specified in a commit log message in a version control system integrated with TeamCity. For example, a developer commits a source code file adding the log message DEMO-5 Added compatibility with older browser versions, the build that includes this change will be shown in Jira in the DEMO project and in issue DEMO-5.