What is the last heartbeat?

The last Heartbeat is the last time that the node reported that it is up and running. Every 5 minutes, each node of the cluster signals that it is up and running and updating it's last heartbeat.

If one node is not updating the last heartbeat for more than 5 minutes, a warning icon will be displayed in the 'Admin' tab -> 'High Availability' page as in the following documentation page screenshot.

Usually, when there is an issue with one of the servers, the state of the server in the HA page is kept in 'Running' state. However, since the server Heartbeat does not get updated for a long interval of time, it is displayed in red with a warning sign.

If you wish to know if your server is currently running, you should use the REST-API calls and ping the node which you think has an issue. 
If you wish to know what was the last time that the server reported that it was running, you can monitor the artifactory_server tables