Tafsir of Al-Mu'minoon 23:3

Al-Laghw (vain talk): That which does not concern you, whether in speech or action, such as play, jesting, and whatever chivalry (muru'ah) dictates should be discarded and cast aside. This implies that they possess a level of seriousness that occupies them entirely, leaving no room for frivolity.

Having described them as having khushu' (humility/focus) in prayer, He followed it with the description of turning away from vain talk. This is to combine for them both the performance (of good) and the abstention (from evil)—the two difficult tasks for the soul that serve as the foundational pillars of religious obligation.