अनुद्वेगकरं वाक्यं सत्यं प्रियहितं च यत्। स्वाध्यायाभ्यसनं चैव वाङ्मयं तप उच्यते ॥

anudvega-karaṁ vākyaṁ satyaṁ priya-hitaṁ cha yat svādhyāyābhyasanaṁ chaiva vāṅ-mayaṁ tapa uchyate

Speech causing no annoyance, truthful, pleasant and beneficial, the practice of the study of the Scriptures, are called the austerity of speech.

In plain words: Self-discipline of speech means saying things that are true, kind and helpful and that do not upset others. It also includes regularly reading and reciting holy books.