I tried to find a thread about this but couldn't...
so:
When you connect resources like cow, stone or Corn to a city, I see this bonus of +1:hammers: or +1:food: in the city screen (in the right side where it shows all resources connected). But when I count my production it doesn't seem to add any more hammers.

What does this Stone:+1:hammers: do?

Nothing; Perhaps it's a reminder that the tile, when worked, produces an additionnal food or hammer, but perhaps also something which does not matter and confuses ore people than it should.