A skill chain refers to using skills in a certain order to cause a desired effect. An example of this could be a A/E Signet Spiker, who uses Dancing Daggers for a lead attack, followed by Blinding Surge and Entangling Asp (which must follow a lead attack) to cause blindness and poison, and finishes up with Signet of Shadows and Signet of Toxic Shock, which exploit both the blindness and poisoned state of the enemy to do massive amounts of damage. These skills are not as effective by themselves, but are powerful when chained together.
"Primer" is a term that can be used to describe a skill that is employed to satisfy the requirements or maximize the effects of another skill (most often an elite skill).
Sometimes two primers are necessary, as is the case when using a skill like Discord.