There are 3 main questions about extensions:

What is extension for chrome?  What it can do? How can we use it?

Basically, extensions are programs that help to improve user experience and customize browser behavior.

Chrome Extensions can be developed using web technologies like HTML/CSS/JavaScript. It is possible to replace a  JavaScript by using any language that in the end can compile to JavaScript(TypeScript, Dart, Elm, etc.).

One of the popular extensions is a Blocking ad from being displayed extension.

Extension detects in the DOM words like ads, advertisement, etc. and removes entire DOM element from the HTML.So, it is also possible to customize the webpage using the extension.

For minimal Hello-World extension development 10-12  lines of code will be needed, but the biggest one will be configuration file (manifest.json) where we should specify what permissions   are needed, where extension should be activated, which file it should run, etc.

The development process is very similar to web development with some nuances. So if you are familiar with Web Development - Then diving into extension development shouldn’t be hard for you, but if you don’t have any experience with web technologies, then it will be better to start from there.

In the end, the extension can be published to Google Extension Store(https://chrome.google.com/webstore/category/extensions?utm_source=chrome-ntp-icon) and everyone can download, install and use your development program in the browser.