It’s just to note that the files are ‘partials’ and are imported from main.less. It’s not actually relevant to Less (although some Less compilers follow this rule…), but it is with Sass:
You can create partial Sass files that contain little snippets of CSS that you can include in other Sass files. This is a great way to modularize your CSS and help keep things easier to maintain. A partial is simply a Sass file named with a leading underscore. You might name it something like
_partial.scss. The underscore lets Sass know that the file is only a partial file and that it should not be generated into a CSS file. Sass partials are used with the@importdirective.
Feel free to remove the underscore prefix if you don’t like it. Looks like Bootstrap might be adding underscore prefixes in v4: Prefix partial LESS source files with underscore · Issue #12902 · twbs/bootstrap · GitHub