As you know <body> tag is very important part of our xhtml or html document as the whole xhtml code comes inside <html></html> so whatever is visible on your browser window that part is inside <body></body> tag.
So whatever you write or insert inside <body></body> tag that will be visible in your browser, simply we can say what visitors see and read , text, images, text inputs, buttons all comes inside <body></body> tag.
We define script files, stylesheets inside <head></head> because those have not to be visible to visitor but script and styles inside <head></head> tag interact with tags in <body></body> tag we will define that in more detail later.
Post your question if you have any.