Java Runtime Environment - JRE

Java Runtime environment of JRE comprises of a Java Virtual Machine (JVM) and Java Core Classes.
While compiling a Java source code, it converts the source code to Java Bytecode. It's a standardized intermediate language and can be run by virtual machines. So the Java Virtual Machine or JVM present on each platform can directly execute the bytecode.
Core classes contains basic methods so that the Virtual machine can call them to execute tasks specified by Java source code which is compiled to Java bytecode.

Suppose, an instruction says to execute a IO operation, the JVM calls a method from the java.io library. This way, a Java Runtime Environment performs.