Benjamin, the ILS (Internal Lock System) is a keyed mechanical locking system that locks the trigger/hammer firing system in place until disabled with said key. Cheers and stay safe!

Internal Locking System (ILS) The lock is a key-operated system that blocks the movement of the hammer when activated. When the key is inserted into the lock and turned counterclockwise, it rotates a cam on the inside of the sideplate. This cam bears upon a rotating locking piece that has a raised stud on its surface, and when the cam rotates, it rotates the locking piece and raises the stud into position which blocks the hammer from moving rearward. The trigger will not move to the rear when the lock is engaged, because the hammer has been fixed in position by the stud on the locking piece https://revolverguy.com/the-history-and-future-of-the-smith-wesson-internal-lock/

The internal lock is like a safety that can't be deactivated without a key. It was mandated to be built into revolvers in the 90s while Bill was in office, it is commonly reffered to as the hillaryhole, many gun owners are not fans of these as they have at times been known to malfunctions and lock up the action making the gun useless, which if that happens in a self defense situation it could have bad consequences.
Chris Baker at lucky gunner has a whole article on it here, but the long and short of it is , their not necessary , they are a liability, and are no longer required by law, so I'd avoid firearms with an internal.lock.
luckygunner.com/lounge/smith-and-wesson-internal-lock/