The form of the class is .is-visible or .is-hidden. On their own, these can be used to show or hide elements, usually using Javascript to unhide them when needed.

.is-hidden {}
.is-visible {}

@-classes can be used to specify if an element's visibility is dependent on screen size.

The @-classes are as follows:

.is-hidden@tiny {}
.is-hidden@small {}
.is-hidden@medium {}
.is-hidden@large {}
.is-hidden@huge {}

.is-visible@tiny {}
.is-visible@small {}
.is-visible@medium {}
.is-visible@large {}
.is-visible@huge {}

It's also possible to target elements with @-classes based on max- and min- sizes. Simply append a + or a - to the class name. For example, .is-hidden@small- has the same effect as .is-hidden@small .is-hidden@tiny.