A chemical barrier
See this: 10 reasons herbicides fail

http://oregonstate.edu/dept/nursery-weeds/feature_articles/top_10/Top_10_reasons.html

Does the chemical barrier burn the tips of emerging roots of weed seeds?

No, they don't "burn" - these types of herbicides are called root inhibitors and impede the steps in plant cell division responsible for chromosome separation and cell wall formation. Roots are relatively few in number and club shaped and unable to support any top growth so the seedling dies a quick death.

See this:

http://www.hostalibrary.org/firstlook/RRPreemergents.htm

The 2nd paragraph says:

"A pre-emergent herbicide is a chemical that burns the roots of all plants in the zone in which it is applied. At normal application strengths, it will burn the tips of emerging roots of weed and other seeds, thus preventing them from growing".

I'd suggest that is a comment from one layman attempting to explain to other laymen in terms they can understand how (some) pre-emergents work. However, the terminology used is inaccurate and misleading.

The science behind how various herbicides work is complex and typically somewhat beyond the grasp of your average gardener :-) Therefore, well-meaning but very simplistic attempts are made to communicate this science in terms that anyone can understand, whether they are accurately descriptive or not. Google "herbicide mode of action" or more specifically, oryzalin or trifluralin (the active ingredients in Surflan and Preen) mode of action and see if you detect the word "burn" in the explanation before your eyes cross from all that chemistry. It ain't there :-))