You are most likely asking about “muhpatti / mopatti” (mouth‑cloth), commonly used in Śvetāmbara (especially Sthānakvāsī / Terāpanth) tradition.

Purpose of Muhpatti (Mopoti)

1. Ahimsā – to avoid killing tiny beings

- When we speak, breathe, cough, or recite sutras, many minute jīvas in the air can be harmed. - Muhpatti reduces this harm by acting as a gentle barrier between our mouth and the outside air.

1. Saṃyam (self‑control) over speech

- With a muhpatti on, we naturally speak less, more slowly, and more carefully. - This helps in avoiding asatya (untruth), ninda (criticizing), krodh (angry speech) etc. - So it supports the vow of satya and overall careful speech.

1. Bhāva of Sadhu‑like discipline

- For monks and nuns it is a regular upkaran (monastic article). - For laypeople, using it during samayik, pratikraman, pāuṣadha, sutra‑pāṭh creates a feeling of humility and reminds us to live closer to sādhujī’s discipline during that time.

1. Smṛti (constant awareness)

- Every time we adjust or feel the muhpatti, we are reminded: “I must not harm even the smallest jīva. I must be alert in every breath and word.” - This continuous awareness itself is dharma‑sādhana.

Sect note (Digambar / Śvetāmbara)

• Śvetāmbara (Mūrtipūjak, Sthānakvāsī, Terāpanth): Muhpatti is a well‑known practice; details of when and how to use it differ slightly by tradition.
• Digambar monks and laypeople: Do not use muhpatti; they follow ahimsā through other strict disciplines (careful walking, limited speech, etc.).

You can read a simple explanation about muhpatti and its purpose here