Disclaimer: I have used bootstrap in the past, but I never really appreciated what it actually is before, this description comes from me coming to my own definition, today. And I know that bootstrap v4 is out, but I found the bootstrap v3 documentation to be much clearer, so I used that. The library is not going to fundamentally change what it provides.
Bootstrap is a collection of CSS and javascript files that provides some nice-looking default styling for standard html elements, and a few common web content objects that are not standard html elements.
To make an analogy, it's kind of like applying a theme in powerpoint, but for your website: it makes things look pretty nice without too much initial effort.
The official v3 documentation breaks it up into three sections:
These roughly correspond to the three main things that Bootstrap provides:
<input class="btn btn-default" type="button" value="Input">Click me</button><span class="glyphicon glyphicon-align-left"></span>Bootstrap extends jQuery quite a bit. If we look at the source code, we can see that it uses jQuery to do things like: set up listeners for keydown event to interact with dropdowns. It does all of this jQuery setup when you import it in your <script> tag, so you need to make sure jQuery is loaded before Bootstrap is.
Additionally, it ties the javascript to the DOM more tightly than plain jQuery, providing a javascript class interface. e.g. toggle a button programmatically. Remember that CSS just defines how a thing looks, so the major job of these operations will tend to be to modify which CSS classes apply to the element at that moment in time. This kind of change, based on user input, can't be done with plain CSS.
There are other standard interactions with a user that we denizens of the internet are used to that are not covered by CSS. Like, clicking a link that scrolls you down a page instead of changing pages. One of the things that Bootstrap gives you is an easy way to implement this behaviour on your own website.
I have mentioned the word "standard" a lot here, and for good reason. I think the best thing that Bootstrap provides is a set of good-looking standards. You're free to modify the default theme as much as you want, but it's a better baseline than raw html, css and js. And this is why it's called "framework".
Different web browsers have different default styles and can act differently, and need different CSS prefixes and things like that. A major benefit of Bootstrap is that it is much more reliable than writing all that cross-browser stuff yourself (you will still have problems, I'm sure, but it's easier).
I think that Bootstrap was preferred more when gulp and babel weren't as popular. Looking at Bootstrap it seems to come from a time before everyone compiled their javascript. It's still relevant, but you can get some of the benefits from other sources now.
More recent versions of CSS have allowed you to define transitions between these static lists as they change. The original version of Bootstrap actually predates wide-spread adoption of this capability in browsers, so they still have their own animation classes. There are a few bits of Bootstrap that are like this: that other stuff has come up around it and makes it look a bit redundant.