help interpreting "explain analyze" output

I'm not really looking for information on how to speed this query
up. I'm just trying to interpret the output enough to tell me which step
is slow:

   Seq Scan on mags  (cost=0.00..187700750.56  rows=47476  width=4) (actual time=3004851.889..3004851.889  rows=0
loops=1)
      Filter:  ((signum IS NOT NULL) AND (NOT (SubPlan 1)))
      SubPlan 1
        -> Materialize  (cost=0.00..3713.93  rows=95862  width=4) (actual time=0.011..16.145  rows=48139  loops=94951)
               -> Seq Scan on sigs (cost=0.00..2906.62 rows=95862 width=4) (actual time=0.010..674.201  rows=95862
loops=1)
   Total runtime: 3004852.005 ms

It looks like the inner seq scan takes 674ms, then the materialize takes
an additional 16ms? Or is that 16ms * 94951? Or 674 * 94951?

And the outer seq scan takes 3004851-3004851 = 0ms?