Class I planets are Earth-like. There are large portions of their surface where it is possible to run around naked without fear of being burnt, frozen, asphyxiated, irradiated or poisoned. It is easy for a large colony to support itself without external supplies.
Class II planets are almost Earth-like. There may be small portions of their surface where it is possible to run around naked without fear of being burnt, frozen, asphyxiated, irradiated or poisoned. At least for a while. Living on such a world long term is a struggle, and may require external supplies.
Class III planets are barely Earth-like. Breathing masks are required, and it isn't possible to farm and settle here without continual support. Running around naked is likely to result in death in a short period of time.
Class IV planets cannot be lived on without an artificial life support system. Trying to run around naked anywhere on their surface will most likely result in death within minutes. Normally they simply lack a breathable atmosphere, or at least one suitable for keeping a human alive. Think Mars.
Class V planets require life support gear beyond a simple pressure suit. Either due to excessive heat, cold or radiation, without specialist survival gear a person is likely to die within a day. Think Mercury or Pluto.
Class VI planets are extremely difficult to even visit without a lot of specialist equipment. They are worlds like Venus where the extreme heat and pressure will crush your landing craft and kill you within minutes. Most gas giants are like this.
A planet's temperature is measured in Kelvin. A night side temperature is also recorded, and will be displayed if it is significantly different from the daytime temperature.