Explain output for Postgres Query

I am trying to understand the Explain output for a query on Postgres. Can anyone explain this output to me?

Merge Join  (cost=0.25..668591683.56 rows=16714280340 width=128)
   Merge Cond: (objref.obj_guid = objhdr1.obj_guid)
   ->  Nested Loop  (cost=0.00..16114.95 rows=165285 width=115)
         ->  Index Only Scan using vomdobjref_idxfk01 on vomdobjref objref  (cost=0.00..6.25 rows=1 width=52)
               Index Cond: ((sys_sid = 0::numeric) AND (prj_sid = 12::numeric) AND (prj_snp_sid = 136::numeric))
         ->  Seq Scan on vomdobjhdr objhdr2  (cost=0.00..14455.85 rows=165285 width=63)
   ->  Materialize  (cost=0.00..2551.65 rows=101124 width=65)
         ->  Nested Loop  (cost=0.00..2298.84 rows=101124 width=65)
               ->  Index Only Scan using vomdobjhdr_pk on vomdobjhdr objhdr1  (cost=0.00..8.39 rows=1 width=52)
                     Index Cond: ((sys_sid = 0::numeric) AND (prj_sid = 12::numeric) AND (prj_snp_sid = 136::numeric))
               ->  Nested Loop  (cost=0.00..1279.20 rows=101124 width=13)
                     ->  Seq Scan on vomdrepcfg repcfg_obj_type  (cost=0.00..7.18 rows=318 width=0)
                     ->  Materialize  (cost=0.00..8.77 rows=318 width=13)
                           ->  Seq Scan on vomdrepcfg repcfg_obj_subtype  (cost=0.00..7.18 rows=318 width=13)