The ScrollRect component is for making the contents of this Slot (such as child objects with RectTransforms) scrollable.
The scrollRect will not do any clipping on its own. If it is a child of an object with a Mask component, it effectively becomes a window into the scrolled content.
It is recommended to use a ContentSizeFitter component and setting the VerticalFit to MinSize when combined with a scrollrect to help fit it within a RectTransform.
This is useful when you have a lot of text or other UIX elements that you want to be scrollable, much like how a webpage has a lot of content that can't fit in just one screen.
Examples
Videos
ProbablePrime's tutorial video on the scrollrect component: