That's not really what convention refers to here. Rails' motto "convention over configuration" simply means that boilerplate should be avoided, and that code should use sensible defaults whenever there isn't information.
For example, ActiveRecord class names map to table names through a simple pluralizer (UserProfile -> user_profiles). That's a convention, no configuration needed. Just write a class and off you go. You can override this behaviour, but you can also write huge apps that just follow the convention. Other frameworks would typically make this mapping explicit.
You do need to read the documentation to see what the conventional behaviour is, but it's not very deep knowledge. It's not much worse than other frameworks, where you need to find how things are configured.
For example, ActiveRecord class names map to table names through a simple pluralizer (UserProfile -> user_profiles). That's a convention, no configuration needed. Just write a class and off you go. You can override this behaviour, but you can also write huge apps that just follow the convention. Other frameworks would typically make this mapping explicit.
You do need to read the documentation to see what the conventional behaviour is, but it's not very deep knowledge. It's not much worse than other frameworks, where you need to find how things are configured.