It's about serenity. Ultimately. But first it's about differentiation: four ways of actively playing with our mind and its impetuous whirling are central (on the subject of 'smiling in the mind'):

Firstly, controlling our immediate thought processes, e.g. by means of re-memory - i.e. repeating affirmations that seem helpful to us. Simultaneously or additionally analysing the origin of our thoughts. (Example: "I smile at myself and accept myself" is one such sentence. At the same time, I realise that my negative self-sabotage wants to make me small so that I am cuddled. Just like my parents did back then)

Secondly, laughing or smiling gently in hindsight at our thoughts and wild minds. Yesterday I thought the world would end if I didn't have these beautiful shoes, this morning I look in the shoe cupboard and very nearly the same ones.

Thirdly, in the moment when the crazy thoughts arise, I smile about them. Technically this means: I look into my mind. this takes some practice because I have to train mindfulness in general.

Fourthly: I look at the mind (dzogchen/mahamudra). Put simply, this "direct view" expands the inner space of the mind. There are various tricks: I look at the third eye from the inside. Or from slightly in front at the top of the mind. Or here is a variation from the Taoist-Buddhist tradition: I physically smile very slightly and relax my forehead.

All four ways of controlling the mind with a smile are in the direction of what is known as gom-pa meditation in the Taoist tradition. gom-pa means "familiarising oneself". Meditation thus means familiarising oneself with the mind. Familiarising oneself is not a dogged practice, but a gentle, smiling way of looking at one's own mind.