A strategy for many fights, staying out of the fire is nonetheless a commonly overlooked habit of players. Depending on the fight, "fire" may be in fact several different kinds of effects (thus also known by the admonition "Don't stand in the bad stuff."), but overall refers to any negative effect to the player that is solved simply by moving. A fight requiring this strategy is usually considered one step up from tank and spank as it only requires paying attention to a second aspect to the fight aside from the target itself. Many players instinctively just try to avoid any of a opponent's spell effects, regardless if they might cause a positive situation.
As these are even more obvious visually than simple debuffs, the phrase usually implies the victim is a complete noob (especially if the effect IS fire, which is always negative) and deserves their fate. Healers waste heals and subsequently hate players who don't stay out of fire, since they can be fatal more quickly than actual bosses
Players with older computers may turn down graphic settings in certain fights and raids to avoid frame rate slowdown, thus making it harder to actually see The Fire. Melee, in particular, tends to either suffer or require them to dance around an enemy to continue attacking. Ranged players are less frequently required to move. Unfortunately for this reason, melee are usually quick to react and have well trained evasion skills, while ranged don't have the habit and thus are more prone to failure. Healer tunnel vision in partiuclar can become a major problem.
Occasionally the effect is either randomized or occurs in a pattern requiring previous knowledge of the fight, and thus is easy or hard depending on the player and their internet connection, such as the Heigan dance.
The Lich Kings ability Defile is a culmination of "Don't stand in the bad stuff". Don't do it. Really.