Ideal developer-weeks (IDW) is defined as the product
of the number of developers  and the time  required to
finish a particular task (or use case coding). e.g. if
you estimate that it should take no longer than 2
weeks of effort with 3 developers, then your IDW is 6
developer-weeks.

The iteration refers to the iteration of development.
RUP's first principle is develop iteratively. i.e. if
you have 10 use cases in a project, identify the max.
risk ones and address thes in the 1st iteration. In
subsequent iterations the other usecases are
handled/coded.

As for estimating the IDW (putting the numbers down),
there are a number of metrices available like function
point analysis, TLOC etc. All of these depend somewhat
on prior experiences with time and developer effort
for previous projects.