Google Signals allows for Cross Device reporting by tracking based off of a cross platform user criteria in addition to a User-ID rather than just a standalone User-ID or Device-ID, whenever applicable.
Google signals allows Google Analytics to take a wholistic view of users that allow for ad personalization. This allows for complete cross-platform tracking on website visits and remarketing ads to users. For instance, Google signals allows for the tracking of a user from their mobile device and desktop as a single user, despite being on two separate devices. Likewise, with remarketing, a user is able to visit a website on their mobile device and receive an ad after leaving, then continue to see those ads when they browse the web from their tablet and again later from their laptop.
Google Signals are enable through GA4 by agreeing to the privacy settings during setup.