Uber will know if you called 911 through its app and will send you a message later to ask if it can help. It won't know what you said during the call, though, and it won't notify drivers (since they may be the reason you called in the first place). The feature is available as part of a larger safety center that also includes information about driver screening, community guidelines and insurance coverage.
International users and drivers will eventually have access to the emergency calling feature, although location sharing is likely to depend on the potential partnerships in the country.
While it's certainly been possible to call 911 from an Uber before, many consider the feature overdue for the US given that a similar option has been available in India for years. Passengers, predominantly women, have long complained of sexual assault and harassment from drivers, to the point where they've filed lawsuits and won settlements. The addition both provides these passengers with a valuable tool and makes sure that Uber is aware of a given crisis far sooner than it might be otherwise.