About

Import permit to add an extern CSS file into a CSS file.

The @import rule allows users to import style rules from other style sheets

You can also import a css file through the HTML link element

This statement has a cost as it seems to make always an an HTTP request.

Syntax

@import "mystyle.css";
@import url("mystyle.css");

Example

Import of bootstrap:

@import url('//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css');

// Add a custom rules here
.container {
    margin-top: 10px;
}
<!-- And we can use the bootstrap CSS rules -->
<button type="button" class="btn btn-info">A bootstrap Button</button>

Documentation / Reference