Java runtime Environment (JRE) is part of JDK. It consists of the Java Virtual Machine (JVM), core classes, and supporting libraries.
Consider a source Sample.java, Compiler converts it into byte code as Sample.class. At run time, Sample.class file loaded into Class loader and Byte code verification will be done in class loader subsystem. Verified byte code will be executed line by line by java interpreter. It converts into machine readable format to processor.
Program which are written by programmer are in .java format lets say sample.java. Once it get compiled, compiler converts .java file to .class file (in the form of byte code) i.e. machine readable form. lets say sample.class.
During program execution, .class file will be loaded to class loader and here these byte code will be verified and executed. As a result, these .class files will be converted into machine language program and it feed to processor.
So over all concept here is to make our human readable program to be converted into machine readable form which respective processor can understand.