make textbox only input number

[MXAlPhA]

I have this example:

Private Sub Text1_KeyPress(KeyAscii As Integer)
    If Not IsNumeric(Chr(KeyAscii)) And KeyAscii <> 8 And KeyAscii <> Asc("-") Then KeyAscii = 0
End Sub

I know this KeyAscii represents the recently inputted character. But i don't get where it comes from.

Hmm... i'm not making any sense. Where does this keyascii come from and how does it work? All i'm asking about is the keyascii variable which i don't understand where it comes from and what it is.