User ID is a term used in Google Analytics, where a specialized report is set up. This type of reports has the particularity of associating a fixed ID of a unique user with the data of the interactions of this person, either in different sessions or different devices.
Usually, in Analytics, we observe the number of sessions or users, but we do not know with certainty how many people they are. If one enters from different devices, the system can take us as different users.
Analytics interprets each user ID as a unique user, this allows us to more accurately identify the behavior of users with our site.
In addition, the view that one sets up with User ID includes cross device reports that are not available in common report views. These types of reports make it easier for us to see and understand user information and behavior more easily and with more specific metrics.
For this type of views we have to make a special configuration both in the Analytics account and on the website, but this work gives us valuable information.
If they have a website and can implement these types of reports it is highly recommended, so do not miss the possibility of doing so.