This module goes hand in hand with the layout module. It serves for telling each .layout__item its width within the grid. As long as it's not .layout--auto, every .layout__item needs a width class.
<div class="layout">
<div class="layout__item u-1/3">
Foo
</div><!--
--><div class="layout__item u-1/3">
Bar
</div><!--
--><div class="layout__item u-1/3">
Baz
</div>
</div>Here, we are telling each .layout__item to take up a width of 1/3 (33.333%).
By default inuitcss creates classes for wholes, halves, thirds, quarters and fifths. You can override the map defined in this module to add more classes.