For many years, scientific inventions have taken notes from naturally occurring biology, planes look like birds, bikes look like horses, trains look like snakes and the list is endless. A less commonly known similarity is our websites. The structure of websites was borrowed from the structure of a human body. A website or web app is divided into three fundamental parts; a header, a body and a footer (by now you notice the similarity).
When writing code that is to be deployed on the web we use HTML (Hyper Text Markup Language) to dictate to the browser the structure of the website you're accessing, HTML produces the skeleton of the website. In it we have things we call "tags". A tag is like the name suggests a way of describing what a bunch of code does, is located or its grouping.
We have a head tag (let's say this is likened to the brain of a human because we declare some very important properties and link the website to files that will give it a more beautiful look). We have a header tag (it's different from a head tag) where we write the code that is at the very top of the webpage, what you find here is usually the logo of the website and navigation links. The body tag has the most content you interact with while the footer is at the very bottom of the web page.
There are other tags which do other things but at the very basic, they're found within the four tags I mentioned. The layout of a website is very important because it helps establish a convention that when followed through results in a better web page. Understanding this layout can also help simplify where things belong.