Muslim mosques usually have an absolution fountain in their courtyard for ritual cleansing. But the Hagia Sophia was originally built as a Christian church. Thus this fountain is located on its west side. Since the Hagia Sophia is now a museum, this fountain is no longer in use.
The columns in front of it are remnants from a previous church built under Roman Emperor Theodosius II. They were unearthed in 1935.