I am now on the bootstrap challenge on FCC now, but I have some questions related to it.
-First, What is bootstrap? (Easy explanation please, because I don’t really get the meaning given by FCC)
-What is the use of bootstrap?
-Where can we get it from?
and can I create it by myself? If so, what I need to know to create one?
If there’s anything wrong with my questions please excuse me, guys.
Thank you in advance for your help
I assume that you are following the current curriculum and not the beta curriculum. The explanation below is taken from the beta curriculum and I think it’s rather well-written (I highlighted the bit that I think is what you’re looking for):
Bootstrap is a front-end framework used to design responsive web pages and web applications. It takes a mobile-first approach to web development. Bootstrap includes pre-built CSS styles and classes, plus some JavaScript functionality.
Paraphrasing the quote above a little, it’s basically a gigantic CSS sheet (and in some cases also a JavaScript file if you need additional functionalities) that you can download (or simply link to one that is hosted on a content delivery network, also known as CDN) and use (for example: <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.3/css/bootstrap.min.css" integrity="sha384-Zug+QiDoJOrZ5t4lssLdxGhVrurbmBWopoEl+M6BdEfwnCJZtKxi1KgxUyJq13dy" crossorigin="anonymous">).
The idea with frameworks like Bootstrap is to give you an bunch of pre-defined, opinionated set of CSS classes so that you don’t have to re-write them every time you work on a new website. For example, applying the classes btn and btn-primary to a <button> element gives you a button with a Twitter-blue background and white text. Another advantage that I can immediately think of is that, for a well-documented and popular framework like Bootstrap, it would be very easy for other developers to understand and work on your code easily.
Please note that there is a drop down menu towards the top right for selecting the version of Bootstrap you want and that the curriculum teaches version 3.3.7. Bootstrap 4 is currently in beta and I recommend using learning the differences once you are comfortable with 3.3.7 (or you could start with Bootstrap 4 but you will have to learn it elsewhere).
You can’t exactly create Bootstrap by yourself because if you “create” one you will either end up copying it or you will create something different that simply isn’t Bootstrap.
There is no shortage of alternatives to Bootstrap (simply search for Bootstrap alternatives) and, to create one, you need to be reasonably familiar with HTML and CSS; for functionality outside of the practical scope of HTML and CSS, you may also want to know JavaScript well.
I hope that helps. 
Thank you @honmanyau very much for your answer. And FCC has 2 types of curriculum? I didn’t know that.
Which curriculum do you think is better, beta or current one?
If beta is better, how can I access that?
@miratorimoolight the beta curriculum has been in the works for a while now and is probably going to be released in the coming months, you can access it at https://beta.freecodecamp.org/en. The beta curriculum is an updated curriculum that is going to replace the current one, I haven’t gone through the entire curriculum myself (I went through only the parts that are new and not in the current curriculum) but my opinion is that it’s much better (and relevant).
Please note that progress is not saved—so if you intend to keep your solutions for the official release, you may want to save your solutions on GitHub or whatever it is that is convenient for you.
I hope that helps. 