What is magento 2 server contracts.
First of all, you should read carefully that you are reading. It is “service contracts”, not “server”.
Secondly, it is just a marketing term, so do not think a lot of it.
http://devdocs.magento.com/guides/v2.0/extension-dev-guide/service-contracts/service-contracts.html
For example. Magento 2 tries to decouple an interface from an implementation:
https://github.com/magento/magento2/blob/e0ed4bad/lib/internal/Magento/Framework/App/RequestInterface.php#L10-L10
In reality, it does not work well (for now):