simple explanation for order-only prerequisite?

[Robert P. J. Day]

  was asked on friday by a make newbie to explain order-only
prerequisite (OOP), as he was reading the explanation in the manual
and was having trouble getting a fix on what it really meant.

  i haven't used one for a while, so i reviewed the section and, AIUI
(and the way i will try to explain it), an OOP means nothing more
than, while the prerequisite will be timestamp checked to see if the
rule needs to be invoked, whether or not that happens is simply not
taken into account for the processing of *this* target.

  in short, while an OOP will be *processed* as usual, its invocation
has no further effect. is that a fundamentally simple (and accurate)
way to describe it? thanks muchly.

rday