It is a component meant to represent a field. `b-` is just the namespace for Buefy components.
I get the feeling that Buefy's approach is to use custom elements instead of native elements? So something like an <input> element becomes some <b-field> thing?
I am sceptical if that is a good thing. I would not want to swap out all regular html for some fantasy elements of a library.
Nevertheless, in the more restricted question of using custom elements to replace CSS classes, you may have a point; many libraries that try to do this typically end up with a far more restricted use case since a custom components library is less open to extension than CSS classes, and custom components authors often fail to anticipate ways in which CSS frameworks are used. (e.g. using a CSS class with a standard HTML tag, and then, using a CSS class with a custom component)
Working with custom "fantasy" elements is quite
typical of modern frontend development
To use a more accurate analogy, they are akin to building an ALU with the use of a an adder (a fantasy component abstracting away the NAND gates) rather than completely in NAND gates.