When we build a navbar, there is a button with class .navbar-toggler which is used to open and collapse the nav-items on mobile devices. A navbar-toggler is a special button since it is not quite actually a button class in bootstrap.
In fact, Bootstrap has a standard class for buttons - .btn . This defines the button object should present itself as a typical button like this:
and it is called by:
The .btn-primary defines the color of the button. There are different colors available such as .*-primary.*-secondary.*-danger etc, where * can be btntextbg etc.
For more detail of Bootstrap colors, you take a look at this Bootstrap Cheatsheet . This cheatsheet is very useful, it shows every classes you may come across.
3. [button sizes]
In addition to colors, there are classes of different button sizes. If you look for Button Modifiers in the cheatsheet, there are .btn-sm , .btn-lg and .btn-block.