Hilite/Hiliting is a bit of (mostly) deprecated functionality in KNIME Analytics Platform. In the past it was used to mark data points of interest in a particular node, so that you could pass those on to other nodes for additional specific analysis. Since the introduction of the Javascript views some time ago, the need for hiliting has mostly gone away, but we still have it present for backwards compatibility reasons.
So long story short, as a new user, you are unlikely to need to use it. Having said that, I am prepared for some KNIME veterans to point out a… differing view.