What is meant by "skim" a caramel?

I am also rather puzzled by it, but I must also say that the recipe is somewhat unusual. There is the dry method for caramel (sugar only) and the wet method (sugar dissolved in sufficient water), I have never seen it done with such slight wetting of water though. So I am not that surprised by the further irregularity.

I can see two possibilities for interpretation here. The first one would be that it is a translation/editing/autocorrect mistake, and that you are meant to stir the caramel. While one expects recipes to be better proofread than that, it can happen, and in this way of doing it, this is indeed the correct time for stirring (and it is a good thing to say so explicitly, so you don't try stirring earlier, which can ruin the caramel).

The second one is that it is indeed meant for skimming foam. Sugar solutions can form foam, which is usually noticeable in jam making. I have never had any significant foam form during caramel making, but it is possible that either the author made it under conditions which somehow produce more foam, or that they believe that the minimal foam formation that likely happens when the water boils off needs skimming. I wouldn't personally see this as an important step, but as I said, the recipe is already unconventional.

In the end, if you feel uncomfortable following these exact unusual instructions, you can either disregard the skimming step, or make a caramel in a way you are comfortable with. By "caramel" I mean pure caramel, not the combination of caramel and cream and/or butter that is also commonly called "caramel" in English speaking countries nowadays. Then proceed with the rest of the recipe as per the book. Or, just find another recipe for caramel ice cream - recipes which are confusingly written are always at a higher risk at failure, either because the author didn't know what they are doing, or because they didn't manage to transfer the knowledge well.