watchOS 5.1.1 new software update was sent to customers by Apple. Some customers were frustrated with previous watchOS 5.1 (released last October 30th) due to brick devices during installation. Although, it was pulled immediately. then working on this fix as soon as possible. With new watchOS 5.1.1 should address this issue. So now watchOS 5.1.1 is included new features, improvements, and bug fixes.
For full information on the security content of Apple software updates, please feel free to check it out on this link.
Source: watchOS 5.1.1 via Apple