Using words that are non-irritating, truthful, endearing and beneficial, and studying scriptures regularly are said to be constituting austerity pertaining to speech.
Austerity is multi-faceted. Human action is in four levels. To begin from outside, we have bodily and sensory activity. This gives rise to bodily austerity. Then comes the oral expression or speech. Thereafter, we come to mental and intelligential activity. These are the four levels of activity, where austerity should be pursued.
Krishna here details the austerity pertaining to speech. Our communication consists of words, sentences, and bhāva (expression). All these should be free from any irritating or offending note. Speech should always be truthful, amiable, evoking fondness and ready reception. Anything to the contrary should be discerned and dispensed with.
Regular study of scriptures and allied texts should be a must for a seeker of spiritual perfection. This is an austerity which keeps our mind disciplined and intelligence clear. Regular association with the pure words and thoughts of the scriptures instils purity and peace in our mind.