What does the s in 6s mean?

The first iPhone with ‘s’ in the model number was the 4S, and at that time it was an ‘S’ for Siri. After iPhone 5, it became a new naming trend, without the original special meaning of the letter.

Edit: @Michael: I stand corrected. I forgot about that model.

It’s whatever you want or think it should mean. Apple to my knowledge never comments on their reasons behind their device model naming conventions. Historically, the “s” models are just larger and/or hardware updated versions of their direct predecessor.

But unless you can find and communicate with a fly on the wall in the room at Apple headquarters, there is no definitive nor official reason for any of the various model designations.

If you want it to mean something then have it mean “second”. The 4 was released, then a 4s (second 4), the 5, then the 5s (second 5), the 6, then the 6s (second 6). As noted, no one here knows the real answer.

There was the iPhone 3Gs before that (although variously people wrote it as iPhone 3GS, iPhone 3G S, iPhone 3Gs and iPhone 3gs).