Execute scalar

I'm trying to use this code but don't understand what ExecuteScalar does despite googling.

userId is returned from a stored procedure. Could someone please clarify?

Protected Sub ValidateUser(sender As Object, e As EventArgs)
        Dim userId As Integer = 0
        Dim constr As String = ConfigurationManager.ConnectionStrings("testConnectionString").ConnectionString
        Using con As New SqlConnection(constr)
            Using cmd As New SqlCommand("Validate_User")
                cmd.CommandType = CommandType.StoredProcedure
                cmd.Parameters.AddWithValue("@Username", Login1.UserName)
                cmd.Parameters.AddWithValue("@Password", Login1.Password)
                cmd.Connection = con
                con.Open()
                userId = Convert.ToInt32(cmd.ExecuteScalar())
                con.Close()
            End Using
            Select Case userId
                Case -1
                    Login1.FailureText = "Username and/or password is incorrect."
                    Exit Select
                Case -2
                    Login1.FailureText = "Account has not been activated."
                    Exit Select
                Case Else
                    FormsAuthentication.RedirectFromLoginPage(Login1.UserName, Login1.RememberMeSet)
                    Exit Select
            End Select
        End Using
    End Sub