language level in Intellij IDEA

The  Language level  setting sets which features the code assistance in the editor should support. For example, if you're using JDK 1.7 but want your code to be compatible with JDK 1.6, you can set the language level lower than your actual JDK supports (6.0 in the case of JDK 1.6) and only get refactorings/syntax suggested that are supported on 1.6 and lower. Depending on your compiler, it may also give the compiler options to remove support for newer syntax elements.
The 8.0 (which, as you're guessing corresponds to Java 8) is available for people that want to experiment with one of the  Java 8 snapshots  that are available. Since Java 8 isn't released, language level 8.0 may very well change before release.
 
편집기에서 문법을 검사하는 level을 지정합니다. JDK7을 사용할 수도 있지만, 6을 호환하고 싶으면 이 level을 6으로 설정하십시오. 6을 초과하면 문법 편집기가 틀릴 수 있습니다.