In Java, what is JVM and how work it ?

JVM stands for Java Virtual Machine.
It is the component, responsible for execution of the Bytecode.
JVM is the one that actually calls the main method present in a java code. JVM is a part of JRE(Java Run Environment).
Java applications are called WORA (Write Once Run Everwhere). this means a programmer can develop Java code on one system and can expect it to run on any other Java enabled system without any adjustment. this is all possible because of JVM.
when we compile a .java file, a .class file with the same filename is generated by the Java compiler.
This .class file goes into various steps when we are run it.