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.
用来指定编辑器检查语法的level，可能你用的JDK7，但是想兼容6，然后就把这个level设置为6的，这样超出6的语法编辑器就会报错。
language level in Intellij IDEA
原文：http://www.cnblogs.com/peterpanzsy/p/4442569.html