CDbl? What is it?

[Uifz]

Hi, I'm trying to create an inputbox for uiJob1TextBox, uiJob2TextBox, and uiJob3TextBox. This way the user will be asked to enter a number for each textbox. Now I have my code:

Const RaisePercentage As String = "Raise Percentage: "
        Dim job1HP As Double, job2HP As Double, job3HP As Double
        Dim job1NewHP As Double, job2NewHP As Double, job3NewHP As Double
        Dim raisePercent As Double
        Try
            job1HP = CDbl(uiJob1TextBox.Text)
            job2HP = CDbl(uiJob2TextBox.Text)
            job3HP = CDbl(uiJob3TextBox.Text)
        Catch ex As Exception
            MsgBox(ex.Message(), MsgBoxStyle.Critical)
            uiJob1TextBox.Focus()
            Exit Sub
        End Try
        Do
            Try
                raisePercent = CDbl(InputBox("Please enter the raise percentage.", "Raise Percentage")) / 100
                Exit Do
            Catch ex As Exception
                MsgBox(ex.Message(), MsgBoxStyle.Critical)
            End Try

I'm trying to understand what CDbl is? Because I'm thinking if i use the 'Try' statement and make job1HP equal to something else that contains in (uiJob1Textbox)
That then I can create a raisePercent= "blah" code which will allow me to enter a InputBox for the three textboxes. Anyone have any suggestions or what
CDbl means and if I can use a different expression so this way I can have expression A equal uiJobTextbox, expression B equal textbox2, and expression C equal textbox3.

Thanks!