As far as I'm aware, the Kudos system allows members to let the author of the post know that their work was appreciated in some way. It's like encouraging someone by giving them a thumbs up. They don't do anything beyond telling you that someone read your work and liked what you wrote.
No. it's not a give and take system.
As long as the giver doesn't withdraw their kudos, then you'll always have their kudo that they gave.
It's just as Gerns said. It's like a "thumbs up" to let the writer know that their post was 'awesome' in some way to the reader and the reader can tell them that "Hey, I loved you're post in the way it was written" through the kudos system. The writer is also encouraged to keep up with that level of writing.