HAR stands for HTTP Archive Format, a text-based format for the log of network communications between a user agent (the browser) and a web server. You can also use this report with a HAR Viewer for in-depth analysis of your JIRA page load performance. (Be aware though that with an online viewer you may transfer sensitive or security-related information to a third party.)
Reload the page by using Ctrl+R or clicking the Reload button. This will make Network tab log all network exchange during page load.
The network tab will start collecting information or network exchange automatically after it's opened. If you know that the problem is not related to the initial page load, you may skip this step to avoid adding extra data to the log. If unsure, reload the page to collect the full report.