So skill chains are basically the GW2 version of auto-attacks. By default, once you select an enemy and hit 1, your character should run through the skill chain automatically. As far as Backstab goes, what I know comes directly from the GW2 wiki: http://wiki.guildwars2.com/wiki/Stealth_skill. As far as I can tell, when you’re in stealth, your first skill slot is replaced by a special Stealth Skill, and once you use that, you’ll be removed from stealth, and your normal combo skill will return. Hopefully this helps!