Just tell me what in this context the words "rational" and "best interest" mean.
Rational means the same thing it means in other contexts. If a rational man is faced with buying the same product for two prices he will chose the lower as a rational choice. That is an example of seeking his "best interest" as well since it gives a greater value to his money.
The assumption of rationality is the same assumption that democracy builds upon.
However, you must consider that the assumption of both requires that there be complete information available about the alternatives.