In plain words
Those established in sattva rise upward; the rājasa remain in the middle; those abiding in the workings of the lowest guṇa, the tāmasa, go downward.
Word for word
Upward go those abiding in sattva; in the middle stay the rājasas; abiding in the workings of the lowest guṇa, the tāmasas go downward.
The commentators
Bhagavadgītābhāṣyam
किञ्च —
ऊर्ध्वं गच्छन्ति देवलोकादिषु उत्पद्यन्ते सत्त्वस्थाः सत्त्वगुणवृत्तस्थाः । मध्ये तिष्ठन्ति मनुष्येषु उत्पद्यन्ते राजसाः । जघन्यगुणवृत्तस्थाः जघन्यश्च असौ गुणश्च जघन्यगुणः तमः, तस्य वृत्तं निद्रालस्यादि, तस्मिन् स्थिताः जघन्यगुणवृत्तस्थाः मूढाः अधः गच्छन्ति पश्वादिषु उत्पद्यन्ते तामसाः ॥ १८ ॥
In plain words
And there's more —
'They rise upward' means they're born among the gods and the higher worlds — these are the ones rooted in sattva, carried by its clear, settled functioning. 'They stay in the middle' means they're born as humans — these are the rajas-natured ones. And 'those rooted in the functioning of the lowest guṇa': that lowest guṇa is tamas, and its functioning is sleep, sluggishness, and things of that kind. Those caught up in it, deluded as they are, 'sink downward', meaning they're born among animals and other lower creatures — these are the tamas-natured ones.
Close to the Sanskrit
Moreover —
'They go upward' [means]: they are born among the gods and so on — those situated in sattva, [i.e.] those established in the functioning of the sattva guṇa. 'They remain in the middle' [means]: they are born among humans — the rajas-natured ones. 'Those situated in the functioning of the lowest guṇa': 'lowest' and 'that guṇa' — the lowest guṇa is tamas; its functioning is sleep, laziness and the like; those established therein — 'those situated in the functioning of the lowest guṇa', deluded — 'go downward' [means]: they are born among animals and so on — the tamas-natured ones.