An external relation is an interpropositional relation which relates the situations described in the propositions by experiential, extralinguistic reality as opposed to relating them by the solely communicative usage of propositions.
If it doesn’t rain, there will be a party at Bill’s house.
This example contrasts with the following sentence, which is an example of an internal relation sequence:
If you’re free tonight, there’s a party at Bill’s house.
This example can be understood as If you’re free tonight, then I offer that there’s a party at Bill's house.