Say you want to keep a resource but you want to stop terraform from managing it. You can ask terraform to update its state to forget about it. In my case I want terraform to forget it managing my Azure Static Web App because Terraform doesn't support all the options I need and will clobber the app.
I can run this
terraform state rm "azurerm_static_site.agentportal"
If I decide to start managing the state again I can just run a terraform import to manage it again.