In Conceptual Metaphor, the source domain is where reasoning happens. Conclusions drawn from the source domain are translated into the Target Domain to solve problems there.
In the figure below, the source domain is the right.
(Double-click to enlarge)
Here, the Target Domain (left) is computer integers, which are numbers that must fit between two boundaries. Nowadays the boundaries are wide: I believe the typical upper boundary is 9,223,372,036,854,775,807. In the past, they were much smaller. I've personally worked on computers where the largest "normal" number was 32,768. (If you knew that was too small, you could use a "long" integer that provided a maximum of about two billion.)
Integer as a Metaphor describes how this is a problem when programmers reason about integers using the source domain of mathematical integers, which have no bounds. (They forget that computer integers – the Target Domain – have an extra constraint.)