In plain words
The Lord dwells in the heart-region of all beings, Arjuna, causing all beings to revolve by Māyā as though mounted on a machine.
Word for word
The Lord dwells in the region of the heart of all beings, O Arjuna, causing all beings, as if mounted on a machine, to revolve by māyā.
The commentators
Bhagavadgītābhāṣyam
यस्मात् —
ईश्वरः ईशनशीलः नारायणः सर्वभूतानां सर्वप्राणिनां हृद्देशे हृदयदेशे अर्जुन शुक्लान्तरात्मस्वभावः विशुद्धान्तःकरणः — ‘अहश्च कृष्णमहरर्जुनं च’(ऋ. मं. ६ । १ । ९ । १) इति दर्शनात् — तिष्ठति स्थितिं लभते । तेषु सः कथं तिष्ठतीति, आह — भ्रामयन् भ्रमणं कारयन् सर्वभूतानि यन्त्रारूढानि यन्त्राणि आरूढानि अधिष्ठितानि इव — इति इवशब्दः अत्र द्रष्टव्यः — यथा दारुकृतपुरुषादीनि यन्त्रारूढानि । मायया च्छद्मना भ्रामयन् तिष्ठति इति सम्बन्धः ॥ ६१ ॥
In plain words
Because —
The Lord — the one whose very nature is to govern, Nārāyaṇa Himself — dwells in the heart of every living being, Arjuna. And you're called Arjuna, 'the bright one,' because your inner self is pure and clear — as the Veda itself distinguishes the dark night from the bright day. But how does He dwell there? Watch: He turns every being as though each one were mounted on a machine — the way a puppeteer turns wooden figures. Notice that 'as if' — it matters. Through māyā, through His own veiling power, He turns them, and in that very turning He abides.
Close to the Sanskrit
Since —
The Lord — one whose nature is to rule, Nārāyaṇa — of all beings, of all living creatures, in the heart-region, in the region of the heart, O Arjuna — one of bright inner-self nature, of purified inner faculty — [Arjuna so named] because of the scriptural statement 'and the day, the dark [night] and the bright [day], Arjuna' (Ṛg. Maṇḍala 6.1.9.1) — abides, attains an abiding. In them, how does he abide? — thus [one asks]; [the verse] answers: causing to revolve — causing revolving to occur — all beings mounted upon a mechanism — mechanisms, ascended, that is, stationed upon — as if; here the word 'as if' is to be noted — as, for example, wooden puppets and the like are mounted upon a mechanism. By māyā — by [His] guise — causing [them] to revolve, He abides: this is the connection [of the sentence].