I don't go out with strangers,"I said."Good thing I do. I'll pick you up at five.

The speaker is saying that they do not typically go out with people they do not know (strangers). However, the other person is saying that it is a good thing they do, as they will be picking up the speaker at 5 o’clock. This implies that the two people have known each other for some time, so the speaker is comfortable with meeting the other person at a specific time and place.