PHP session
Japanese PHPセッション
A PHP feature that briefly remembers information such as login status on the server while a visitor moves between pages. It is opened with session_start(), and WordPress Site Health flags an open session as a critical issue.
A PHP session is a feature that lets the server briefly remember things about a visitor, such as whether they are logged in or what is in their cart, while they move from page to page. On their own, web pages open one at a time and do not remember what happened on the previous page. A session is like a shop handing a customer a numbered ticket and using that number to remember their order. In code, a session is opened with session_start().
Watch out If a plugin or other code leaves a session open, WordPress shows "An active PHP session was detected" as a critical issue on the Site Health screen. If you see this, check the plugins you installed recently, and ask the plugin's developer or whoever manages your site if needed.
Source: WordPress Developer Resources, "get_test_php_sessions"