The unbanked Americans are people who either are unable to open an account, or have been denied opening accounts. To cash their checks, keep their income, etc..., they have to pay much more in fees.
Historically (in the U.S.) it was rooted in racism post-U.S. civil war. Now, less so. The history is still there, and the inability of poor people to obtain an account still exist. It excludes people from getting mortgages, loans, investing, etc. Cash App has become a digital bank for many of the unbanked. Before Cash App, decades ago, we had the U.S. Postal Service providing some banking services to Americans. (No, the USPS attempting to help Americans in this manner is not new, we've done it before)