In succession

To use skills in succession means to queue two (or more) actions (skills on the skill bar or actions) by invoking one then the other (while the first one is still activating in the case of skill). The technical difference maker is whehter the second action was done immediately after the character finished the cooldown animation of the first skill or was there a separation (even a fraction of a second) where by the character stands upright again.

Using area of effect (AoE) skills in succession has an effect on how the AI of computer-controlled monsters will respond. It is one of the determining factors of how these monsters decide if they will flee an AoE attack or not.

For example, if a player casts Fireball or Rodgort's Invocation on computer-controlled enemies, they will not flee. But if that player casts them both in succession, the same two spells will cause the enemies to flee. If the player waits even the smallest fraction of a second before queuing the second spell, the monsters will not flee.

The same applies to a spell like Zealot's Fire or Mark of Pain which fires off AoE damage when a condition is met. If that condition is met through actions done in succession then it will trigger off the fleeing AI in the monsters. For Zealot's Fire that would be casting two spells in succession, for Mark of Pain, that would be making two attacks on the hexed target in succession.