JRE

JRE is the core of Java - it stands for Java Runtime Environment.

To understand JRE, I will give an analogy. Consider Adobe Acrobat and a pdf file.
Adobe Acrobat is a program that I can install on my computer. I can also install it as a plugin in my browser, or as an App on my smart phone.

Once Adobe Acrobat is installed, I can open any pdf file in it. I can view the pdf file, copy-paste from it, print it, etc.

JRE is also a program, just like Adobe Acrobat. It can be installed on any computer, it can be installed as a plugin in browsers, it can be installed on smart phones, even on devices like microwaves and refrigerators (smart home!).

Once JRE is installed, it can run any Java program (just like Adobe Acrobat can open a pdf file).