Can someone explain to me the difference between roles and Users in postgresql

As I ask in stackexchange https://dba.stackexchange.com/q/325711/118215 whilsed I was looking in this answer https://dba.stackexchange.com/a/177272/118215

I noticed that Roles is something different from Users. But I cannot understand the difference. Can someone explain to me why roles are different from users and what the use cases for each?