In plain words
The space between heaven and earth and all directions are filled by You alone. Seeing this wondrous and terrible form of Yours, the three worlds tremble, Mahātman.
Word for word
For this space between heaven and earth [is] pervaded by You alone, and all the quarters; seeing this wondrous, fierce form of Yours, the three worlds [are] shaken, O great-souled one.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
द्यावापृथिव्योः इदम् अन्तरं हि अन्तरिक्षं व्याप्तं त्वया एकेन विश्वरूपधरेण दिशश्च सर्वाः व्याप्ताः । दृष्ट्वा उपलभ्य अद्भुतं विस्मापकं रूपम् इदं तव उग्रं क्रूरं लोकानां त्रयं लोकत्रयं प्रव्यथितं भीतं प्रचलितं वा हे महात्मन् अक्षुद्रस्वभाव ॥ २० ॥
In plain words
The whole stretch between earth and sky — 'the interval', which means the open mid-air — is filled by you, and by you alone, wearing the form of everything there is; and every direction is filled as well. Having seen, that is, having actually laid eyes on, this form of yours — wondrous, meaning it stuns the mind that looks at it — and fierce, meaning savage, the three worlds together are greatly agitated: frightened, or else shaken loose from their places. O great-selfed one — you in whose nature there is nothing small.
Close to the Sanskrit
This interval of heaven and earth — indeed, the mid-region — is pervaded by you alone, the bearer of the universal form; and all the directions are pervaded. Having seen — having perceived — this wondrous, astonishment-causing form of yours, fierce, that is, cruel, the triad of the worlds — the three worlds — is greatly agitated, that is, frightened, or else shaken from its place, O great-selfed one, O you whose nature is not petty.