This is the compiler from Scala to Java bytecode. It does type checking, some optimizations (like inlining, removing some instances of boxing, or converting tail-recursive functions to loops), and converts all Scala-specific features to some Java constructs (e.g. traits become interfaces, classes etc.; lazy values become methods and accompanying values; lambdas get lifted to the class they are defined in).

The JIT works on Java bytecode. Java also has a similar compiler (javac) from Java -> bytecode.

They are talking about compilation of source code to Java byte code, i.e. the same kind of compiler as javac from the JDK. (There is also Scala Native, but that doesn't seem to be what this is about.)