One is Google's new cloud-storage solution for those who formerly used services such as paid Drive. It provides a unified location for the data generated by apps such as Photos, Drive and Gmail. Google One has been absorbing Drive customers who wish to switch to it, and is now available to new customers too.
https://www.notebookcheck.net/Google-s-new-cloud-service-has-come-into-effect.323494.0.html
Did Google really have to use the name "1" for a cloud storage service? Microsoft already has "OneDrive".