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?