In plain words
As a lamp in a windless place does not flicker—this is the image given for the yogin of controlled mind who practises yoga on Ātma.
Word for word
As a lamp placed where there is no wind does not flicker, that is held [to be] the image of the yogin whose mind is controlled, practising yoga on the Self.
The commentators
Bhagavadgītābhāṣyam
तस्य योगिनः समाहितं यत् चित्तं तस्योपमा उच्यते —
यथा दीपः प्रदीपः निवातस्थः निवाते वातवर्जिते देशे स्थितः न इङ्गते न चलति, सा उपमा उपमीयते अनया इत्युपमा योगज्ञैः चित्तप्रचारदर्शिभिः स्मृता चिन्तिता योगिनो यतचित्तस्य संयतान्तःकरणस्य युञ्जतो योगम् अनुतिष्ठतः आत्मनः समाधिमनुतिष्ठत इत्यर्थः ॥ १९ ॥
In plain words
For that yogin's mind, gathered and settled, a comparison is now given —
A lamp — just a lamp — set down where no wind reaches it, in a place with the air completely still: it does not waver, it does not move at all. That is the comparison the verse gives; and 'comparison' means simply the thing by which one likeness is measured against another. The teachers who know yoga handed this image down — thought it through and set it there — and they are the ones who have actually watched how the mind ranges about. The image is for the yogī whose mind is held in, whose whole inner instrument is gathered and controlled, who is yoking himself, that is, doing the practice: fixed on the Self. In other words, one who is settled in samādhi. That is the meaning.
Close to the Sanskrit
Of that yogin, the mind which is collected — of that a simile is stated —
Just as a lamp — pradīpaḥ, a lamp — standing in a windless place, that is, situated in a place devoid of wind, does not stir, does not move: that comparison — upamā, since it is that by which a thing is compared, hence 'upamā' — is recalled, that is, was thought out, by the knowers of yoga, that is, by those who have seen the ranging of the mind, with respect to the yogī whose mind is restrained, that is, whose inner instrument is wholly controlled, who yokes himself, that is, who carries out yoga; of the Self — the sense being, of one who is practising samādhi. This is the meaning.