It creates the environment to develop the Java program.
It contains development tools such as Java compiler (javac.exe), Java application launcher (java.exe), Java debugger, Applet viewer, etc
It is platform specific so you need to use separate installer for Linux, Windows.
When you install JDK, installation folder is often called as JAVA_HOME (root).
The JAVA_HOME/bin contains Java compiler, Java debugger, other resource binaries, and etc.
In order to compile and execute the Java program, JAVA_HOME path should be mapped to Java source file (.java) by Java classpath method.