Pre-Update Check
From Joomla! Documentation
New to Joomla! 3.10.x is the Pre-Update Check component. This component shows technical specifications of the server the site is on and core and third-party extensions that use the Update Server in a list form.
Technical Specifications
The top portion of the Pre-Update Check shows if your current server environment is compatible with Joomla 4.x for both the required PHP and Database Settings. The data on the Technical Requirements page will be updated to show Joomla 4.x specs when we are closer to GA.
Your site may look different than the screen capture above. What's important is to check if anything is red and is not compatible with Joomla 4.x. If it isn't, you'll need to talk to your host or change hosts. During a migration is a great time to change hosts. See the Migration Funnel for more information on planning and step-by-step instructions (docs from 3.10.x to 4.x aren't ready yet. Use the 2.5 to 3.x funnel as a guide.)
Extensions Pre-Update Check
The extension portion of the Pre-Update Check component pulls Joomla core extensions and third-party extensions that use update server.
The list is broken into sections based on the targetplatform tag the extensions are using. See Deploying an Update Server for more information about tags.