"Infirm special pioneer" is someone who has served as a special pioneer for many years, and has had their health ruined as a result.
In their utter magnanimousness and grace, the Watchtower Society will allow them to continue to have the designation "Special Pioneer" even though they don't have the strengh to make the full 120 or 130 hours of field serve-us required.
Most importantly, by keeping the "Special Pioneer" name, they continue to be entitled to the below-poverty wages (currently $600/month in the US) and free health insurance the Society provides.
I'm not sure what the age & service requirements are now - a few years ago, you had to be at least age 55 with 15+ years of "full time service" to be allowed to keep the "regular pioneer" designation (with of course no perks at all) and turn in reduced hours. Since the Society is (reluctantly) spending its own cash for the "infirm special pioneers", I imagine the requirements are far more restrictive.