All wool tallitot katanim I have ever seen have reinforced corners, usually made from a square of cotton cloth. This prevents the tzitziyot from tearing through the hole from daily "wear and tear" and rendering the tallit katan passul (unusable).
However, aside from the usual differences that distinguish a Chabad tallit katan from others (two diagonal holes; tzitzit tying mehtod; round neck), a Chabad tallit katan has reinforced corners with silk squares, rather than cotton. I have been told this is the Chabad minhag. I am inclined to believe this is specifically a Chabad minhag, as I have never seen a non-Chabad tallit katan with silk corners, only with cotton corners.
Why is the Chabad minhag to have corners reinforced with silk? In the comments to this question before I edited, people didn't understand that I know that the silk is there to reinforce the corners and keep the hole from tearing through. But why specifically silk, when other tallitot katanim have cotton corners? Cotton seems to me a better material to reinforce with anyway, as silk is a delicate cloth and can easily develop tears, holes, etc.
For those who were unfamiliar with this aspect of a Chabad tallit katan, please refer to these pictures, courtesy of Ben's Tallit Shop:
As you see above, the Chabad tallit katan has silk corners...
...Whereas a regular tallit katan has cotton corners.