Because this has been discussed at such great length, I'll give the abridged version: The internal lock is a mechanism built into most newer (2001-date) S&W handguns which, when activated with a provided key, disables the action of the gun and prevents it from firing. While present in both revolvers and semi-automatic pistols, the revolver version seems to be the most controversial. A lock-equipped revolver can be easily recognized because of the small keyhole just above the thumbpiece.
The lock itself was born in controversy and continues to be a point of contention with many people today. A locking mechanism of some sort was part of the infamous "Agreement of 2000" that S&W under their previous owners, Tomkins PLC, entered into the Clinton Administration with though the lock that S&W actually incorporated does not technically meet the terms of the agreement since they do not use individualized keys (a key from one ILS S&W will work in any other). The lock was not actually incorporated by Tomkins, but rather by the subsequent owner, Saf-T-Hammer, to whom Tomkins sold S&W at a considerable loss due in no small part to the fallout from the Clinton agreement.
Some people continue to view the lock as the product of the Clinton Agreement and thus dislike it because they consider it a "sellout" to gun control advocates and thus resent what it represents. Still others dislike the lock because they feel that it detracts from the aesthetics of S&W handguns. Both of those views cannot really be argued because they are purely matters of personal opinion and taste.
The largest point of contention about the lock, however, is to what degree, if any, it affects the reliability of the gun. There have been a very small number of documented cases in which the lock has caused the action of the gun to seize up under recoil without being intentionally activated by the shooter. While the number of documented cases is quite small, there are many more cases of claimed "auto lock" which cannot be verified. Some claim that "auto locks" are fairly common while others, myself included, believe that they are very rare and isolated incidents and not frequent enough to be concerned with.
Another point often made by detractors of the lock is that, even if "auto lock" is exceedingly rare, the fact that it is a non-zero probability is unacceptable and therefore ILS guns should be either avoided or have their locks disabled if they are to be used for "serious" purposes like self-defense. Others, myself included, contend that the incidence of "auto lock" is, in fact, so rare that it is no more likely to cause a S&W handgun to malfunction than any of the other parts which could potentially be broken or defective.
As a final word of caution, I warn you that discussions about the ILS frequently become less-than-civil. More often than not, discussions about the lock digress to the point that childish invectives like "Safey Wesson," "Smith & Clinton," or "Hillary Hole" are thrown about and people are accused to being "apologists" for their view. That's not to say that you can't or shouldn't discuss the issue, just be aware that some who will likely join the discussion won't be reserved or polite about expressing their opinions.