Welcome to Testrail.  Once you are logged in, you need to link your Testrail account to your Jira account. To do this, click your user name in the upper right corner of the UI. Click My Settings and then the Settings tab.  Enter your Jira username and password in the fields.  (Note: there are fields for two Jira accounts but most of us only have one.)   After that, you're done.  You can now see and add test cases from Jira as well as seeing the results of test runs!

Happy Testing!