मनःप्रसादः सौम्यत्वं मौनमात्मविनिग्रहः। भावसंशुद्धिरित्येतत्तपो मानसमुच्यते ॥

manaḥ-prasādaḥ saumyatvaṁ maunam ātma-vinigrahaḥ bhāva-sanśhuddhir ity etat tapo mānasam uchyate

Mental happiness, equilibrium, silence, self-control, purity of nature—these are called the austerity of the mind.

In plain words: Self-discipline of the mind means keeping a calm, cheerful mind. It also means being balanced, quiet inside, in control of yourself, and pure in heart.