Combo is short for combination. In some cases when you use a special attack/ability, you will have the possibility to use a continued attack/ability, that relies on the first. This often results in more damage on less effort or prevents the use of strongest abilities in the beginning of a fight.
Classes using mechanics of this kind are:
Additionally other classes have single abilities, that require a specific effect to be active on the target to use an additional effect. The warrior priest's slow would be an example of that behaviour.