Hot and cold states refer to periods of high or low arousal. This can be caused by a perceived threat, a very large reward, or a highly desirable "super" stimulus (e.g. a vivid video of a pizza, a scantily-clad human, etc). People tend to behave differently in hot and cold states, and also underestimate the difference between their selves in each.
Attempts to influence behavior using this principle tend to aim to shift someone from a hot to a cold state, or vice versa. There is some evidence that people use more automatic thinking (sometimes referred to as "System 1") during hot states, and more deliberate thinking ("System 2") during cold ones.