In plain words
Serenity of manas, gentleness, silence, restraint of the mind, and purity of intention—this is called tapas of manas.
Word for word
Serenity of mind, gentleness, silence, restraint of the mind, purity of disposition: thus this is called tapas of the mind.
मनः प्रसादः सौम्यत्वं मौनमात्मविनिग्रहःभावसंशुद्धिरित्येतत् तपो मानसमुच्यते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
मनःप्रसादः मनसः प्रशान्तिः, स्वच्छतापादनं प्रसादः, सौम्यत्वं यत् सौमनस्यम् आहुः — मुखादिप्रसादादिकार्योन्नेया अन्तःकरणस्य वृत्तिः । मौनं वाङ्नियमोऽपि मनःसंयमपूर्वको भवति इति कार्येण कारणम् उच्यते मनःसंयमो मौनमिति । आत्मविनिग्रहः मनोनिरोधः सर्वतः सामान्यरूपः आत्मविनिग्रहः, वाग्विषयस्यैव मनसः संयमः मौनम् इति विशेषः । भावसंशुद्धिः परैः व्यवहारकाले अमायावित्वं भावसंशुद्धिः । इत्येतत् तपः मानसम् उच्यते ॥ १६ ॥
In plain words
'Serenity of mind' means simply: tranquility of the mind. The bringing about of that clarity is what 'serenity' really is -- also called gentleness, or good cheer -- and you know it is there by its effects, by the clarity that shows on someone's face and the rest; it is really a state of the mind itself, inferred from what it produces. 'Silence' means restraint of speech too, but that restraint always comes preceded by control of the mind -- so really, by naming the effect, what is being pointed to is the cause: mental control itself is what deserves the name silence. 'Self-restraint' means control of the mind in a completely general sense, applying everywhere. The difference is this: silence specifically means controlling the mind where speech is concerned. 'Purity of intention' means, in your dealings with others, simply being free of any deceit. All of this together is what is called mental austerity.
Close to the Sanskrit
'Manahprasada' [means] tranquility of mind; 'the bringing about of clarity' [is] 'prasada'; 'saumyatva,' which they call 'saumanasya' -- [this is] a state of the inner organ to be inferred from effects such as clarity of the face etc. 'Mauna' [means] restraint of speech too, [which] arises preceded by control of the mind; thus, by the effect, the cause is being expressed: 'control of mind is called mauna.' 'Atmavinigraha' [means] restraint of mind, of a general nature; [that is] 'atmavinigraha.' The distinction is: 'mauna' [refers] specifically to restraint of mind with respect to the object of speech. 'Bhavasamsuddhi' [means], toward others, at the time of dealing [with them], freedom from deceitfulness -- [that is] 'bhavasamsuddhi.' This is what is called mental austerity.
▸Gītā Vāhini
“మనసున ఎట్టి విచారమును పెట్టుకొనక సకల ప్రాణులకు ఎల్లప్పూడూ మేలుకోరుచుండవలెను. పరమాత్మను మననము చేయుచుండవలెను. ఇతర విషయముల పై పరుగెత్తు చిత్తమును స్వాధీనమందుంచుకొని, పవిత్ర భావములను మాత్రమే మనసున కలిగియుండవలెను. ఇట్టి దానిని మనో తపమని అందురు.”
In plain words
“Keeping no worry at all in the Manas, one must always wish for the good of all living beings. One must keep contemplating the Paramātma. Holding in check the citta that runs after other things, one must hold only holy feelings in the Manas. This is called the tapas of the Manas.”