Tab Discarding is a feature of Chromium-based browsers where the browser unloads a tab which has not been focused by the user for a certain amount of time and reloads it upon entry when the user focuses the tab again.
It is intended to save system memory and improve performance on low-powered hardware.
In my experience, it is extremely annoying, as it will unload web-based versions of resource-intensive (read: unoptimized) applications such as Slack, Teams, Outlook etc.
On top of that, it does not save that much memory and will lose page state with potentially unsaved changes in it. It's a pain in the ass and an anti-feature from my point of view, so I am always eager to disable it on all my devices running Chromium-based browsers.
To disable it, go to:
chrome://flags/#automatic-tab-discarding
Switch the value to Disabled and restart Chrome.