In vb.net, what does the ! operator after DataRow do?

Question

Everyone, what does the "!" after the DataRow Object means? And why DataRow can get ColumnName Attribute?

Sub constructAttrMeta(ByVal objRows As DataRow)
        Try
            setColName(objRows!ColumnName.ToString)
            ...
        Catch ex As Exception
            Throw New Exception
        End Try
End Sub


Sub setColName(ByVal sColName As String)
        Me.m_sColName = sColName.ToUpper
End Sub