They mean sensitization. It’s where you react more strongly to exposure the more you’re exposed. It’s a problem for a couple of reasons. The scary one is some people move along the reaction continuum from “mildly annoying” to “oh shit call an ambulance” pretty fast. It’s not common but it happens. I actually have a condition that predisposes me to that so I always carry an epi-pen. Good times.

The more mundane one is, even with controls to remove fumes and PPE to avoid them, it’s not uncommon to still be exposed to a small degree, especially in a hobbyist setting. Let’s imagine you’re lax on avoiding exposure and become sensitized due to prolonged and/or heavy exposure, so you set up proper controls to try to avoid it. Now the small amount of remaining exposure which may have never been a problem with proper controls may actually become a problem. Now you can’t work with that material at all without having some sort of reaction.