In plain words
These hosts of gods enter You; some, frightened, praise You with joined palms. Hosts of great seers and siddhas cry, 'May all be well!' and extol You with abundant hymns.
Word for word
For these hosts of gods enter You; some, afraid, with joined palms, praise [You]; saying 'May it be well', hosts of great sages and siddhas praise You with full hymns.
The commentators
Bhagavadgītābhāṣyam
अथ अधुना पुरा ‘यद्वा जयेम यदि वा नो जयेयुः’(भ. गी. २ । ६) इति अर्जुनस्य यः संशयः आसीत् , तन्निर्णयाय पाण्डवजयम् ऐकान्तिकं दर्शयामि इति प्रवृत्तो भगवान् । तं पश्यन् आह — किञ्च —
अमी हि युध्यमाना योद्धारः त्वा त्वां सुरसङ्घाः ये अत्र भूभारावताराय अवतीर्णाः वस्वादिदेवसङ्घाः मनुष्यसंस्थानाः त्वां विशन्ति प्रविशन्तः दृश्यन्ते । तत्र केचित् भीताः प्राञ्जलयः सन्तो गृणन्ति स्तुवन्ति त्वाम् अन्ये पलायनेऽपि अशक्ताः सन्तः । युद्धे प्रत्युपस्थिते उत्पातादिनिमित्तानि उपलक्ष्य स्वस्ति अस्तु जगतः इति उक्त्वा महर्षिसिद्धसङ्घाः महर्षीणां सिद्धानां च सङ्घाः स्तुवन्ति त्वां स्तुतिभिः पुष्कलाभिः सम्पूर्णाभिः ॥ २१ ॥
In plain words
Earlier Arjuna had said: 'whether we win, or whether they defeat us' — and that doubt had stayed with him. Now, to settle it, the Lord has set about showing him the Pāṇḍavas' victory as an absolute certainty. Seeing Him at this, Arjuna speaks — And further —
Look — these very warriors fighting before you are gods, hosts of them, who came down here in human form to lift the crushing weight from the earth, and now I see them entering into you. Some, terrified, palms pressed together, cry out in praise; others are so undone they cannot even run. And just as sages and seers watch for omens before a battle and murmur, 'may the world be safe,' so now the great sages and the perfected ones gather and pour out their full, overflowing hymns of praise to you.
Close to the Sanskrit
Now, at present — that doubt of Arjuna which existed formerly, 'whether we should conquer, or whether they should conquer us' (Gītā 2.6) — for the settling of that, the Lord has engaged upon this: 'I shall show the victory of the Pāṇḍavas to be certain.' Seeing Him, he says — Moreover —
These indeed, fighting warriors, you — the hosts of gods who here, for the descent to relieve the earth's burden, have descended, groups of gods headed by the Vasus, in human form — enter into you: they are seen entering. There, some, frightened, with joined palms, being so, proclaim, praise, you; others, being unable even to flee. When battle is imminent, having observed portents and the like, having said 'may there be well-being for the world,' the assemblies of great sages and siddhas — the groups of great sages and of siddhas — praise you with hymns, abundant, complete.