Learning to read explain

Any good sources on how to properly interpret explain?

For instance I started out with something like:
Hash Join (cost=20279.05..128002.12 rows=1064 width=78)
-> Seq Scan on yhearn yhe (cost=0.00..23058.65 rows=1175565 width=38)
-> Hash (cost=18659.62..18659.62 rows=156970 width=40)
-> Seq Scan on hearn he (cost=0.00..18659.62 rows=156970 width=40)

After looking at my tables and what would be my most restrictive
conditions I got down to:
Nested Loop (cost=0.00..65999.17 rows=21 width=78)
-> Seq Scan on yhearn yhe (cost=0.00..28936.47 rows=7655 width=38)
-> Index Scan using he_horse on hearn he (cost=0.00..4.82 rows=1 width=40)

But it would have helped if I better understood what is the "cost".
Also what is the "Hash" on the first version of my not-optimized query.