An ICU is usually a deli cup with moist paper towels, so very high humidity and generally kept at higher temperature. If a tarantula has any issues (say they drop a limb or have a bad molt, or are in any other kind of condition that would require healing) then the conditions are better to keep them hydrated while they heal and the higher temperature allows them to speed up their metabolism and hopefully the healing process while keeping any kind of open wound clean due to there being no loose substrate.
I don't own T's so that really had me stumped! Thanks
0.1 Normal - Suni
1.0 Spider - Crank
0.1 Pinstripe - Tielle
0.0.3 Normal - Meatball, Larry & Nautis