Temporary conditions are categorized just like regular conditions as fleeting, sticky or lasting, with appropriate stress values. The biggest difference is that these conditions are not available for the player to choose to take, such as to absorb stress - they only come into existence as the result of specific circumstances. Stunts are one way that temporary conditions can be created - some stunts may cause them, while others provide a character with a condition (which equals being able to absorb more stress than a normal character). A barbarian stunt called Rage might give the character the sticky condition Berserker with the requirement that the character must absorb at least 4 stress before marking the condition off (2 for the Berserker condition and another two as the "trigger"). Temporary conditions may also come into play when it makes narrative sense. For example, stress inflicted by a blizzard that's been given a Winter Storm skill to attack characters might result in the condition Frostbite. Depending on the exact type of condition, the temporary one may replace or make unavailable one of the predefined conditions. Starving might replace Exhausted, or Possessed might make Broken unavailable until it is cleared. Not all of the stress has to go toward the temporary condition either - a mental attack that paralyzes an opponent might deal 4 stress, giving the target the temporary sticky condition Paralyzed and leaving them mentally Bloodied.
In short, temporary conditions are meant to fill in the gaps when using conditions by allowing "on demand" conditions to be written in. Some care has to be taken to not make it a complete zero-sum game (otherwise, they aren't much different than normal aspects), so they are best if used when they are going to be on the character sheet for a while (at least an entire session).