SLS stands for sodium lauryl sulfate, which is widely known in toothpastes to be a tissue irritant, particularly problematic if your have herpes simplex lesions, or aphthous ulcers in your mouth. HSV1 lesions and aphthous ulcers are very common throughout the US population so having this ingredient in a paste is not a great thing.

It's essentially a detergent, but there are many others to choose from that are far less irritating. It chosen often in cosmetics, shampoos, toothpaste etc. because it is very inexpensive. There are some companies that attempt to hide the fact that it is a chemical to be avoided, some go so far as to suggest it is a "natural" product (whatever that means today as it is a flimsy word tool to get people to trust things they buy) because it comes from coconuts. To anyone unaware this seems like it should make it ok to include in "natural" products. Having said that, many "naturally" occurring products in the world are carcinogenic, poisonous, hormone altering, (I could go on ad nauseum here), and essentially toxic. Manufacturers that use it say that yes, there are thousands of negative scientific articles on SLS, but it is an issue of "volume and frequency of exposure"... While true, I choose to avoid it, life has enough things in it that are already assaults on my biology, why add to the burden.

If you could choose a dentifrice that did not contain this ingredient, it would be a better choice than one that does.