Unable to understand this jquery code [closed]

Your specific questions:

var featureIconHolder = $(".feature-icon-holder", "#features-links-holder");

Here’s the documentation link for this: http://api.jquery.com/jQuery/#jQuery1
The two arguments here are selector and context. What this does is find all elements of class feature-icon-holder that are descendents of the element with ID features-links-holder. This constructs an array-like object that contains 0+ elements depending on how many matched.

what is $(this).data("id")

Inside the click handler, this is the element that has been clicked. So $(this) is a jQuery object for that element, and .data reads or stores values against an element. It’s then used to construct another selector by concatenating strings.

---

This changes the displayed ‘feature’ based on the icon clicked. What it does:

1. Find all elements with class feature-icon-holder inside the element with ID feature-links-holder.
2. Set up a click handler on all of these elements:
   1. removes the ‘opened’ class from all of these icons, i.e. it clears ‘opened’ from the previously open element. The opened class is probably a highlight around the icon.
   2. sets the ‘opened’ class on this icon, presumably highlighting it.
   3. clears the ‘show-details’ class from any elements inside ID feature-holder that currently has the class; again there’s probably only one of these, the currently displayed feature
   4. adds the ‘show-details’ class to the element inside feature-holder that has a class name matching the ID stored with the icon clicked. This will presumably show the feature elsewhere on the page.