Obviously this can't mean the "auto-attack" that occurs when you are standing idle and press a Combat Art to stab a mob, and you automatically engage combat attack mode.
I think you may be hitting on it here, auto attack is when you engage a mob and do not use any CAs.
It is started by one of the first CAs you get, prolly got it while on the initial boat ride (if you went on the first boat ride).
This art makes you swing your weapon or use your bow, depending on which one is keyed.
The weapon auto attack has a picture of crossed swords, I think, and the bow auto attack has a picture of a bulls eye if I recall correctly.
When using auto attack the damage you do will be based on the damage rating/stats of the weapon you are using in addition to any procs if your weapon is imbued or has any proc affect noted.