In plain words
With the self unattached to external contacts, one discovers the happiness that is in Ātma. With the mind collected in yoga on Brahman, one enjoys inexhaustible happiness.
Word for word
One whose self is unattached to external contacts finds the happiness that is in the Self; he, whose mind is collected in yoga on Brahman, attains imperishable happiness.
The commentators
Bhagavadgītābhāṣyam
किञ्च, ब्रह्मणि स्थितः —
बाह्यस्पर्शेषु बाह्याश्च ते स्पर्शाश्च बाह्यस्पर्शाः स्पृश्यन्ते इति स्पर्शाः शब्दादयो विषयाः तेषु बाह्यस्पर्शेषु, असक्तः आत्मा अन्तःकरणं यस्य सः अयम् असक्तात्मा विषयेषु प्रीतिवर्जितः सन् विन्दति लभते आत्मनि यत् सुखं तत् विन्दति इत्येतत् । स ब्रह्मयोगयुक्तात्मा ब्रह्मणि योगः समाधिः ब्रह्मयोगः तेन ब्रह्मयोगेन युक्तः समाहितः तस्मिन् व्यापृतः आत्मा अन्तःकरणं यस्य सः ब्रह्मयोगयुक्तात्मा, सुखम् अक्षयम् अश्नुते व्याप्नोति । तस्मात् बाह्यविषयप्रीतेः क्षणिकायाः इन्द्रियाणि निवर्तयेत् आत्मनि अक्षयसुखार्थी इत्यर्थः ॥ २१ ॥
In plain words
And there is more to say — of the one who stands established in Brahman:
'In external contacts' — they're called 'external contacts' simply because they're touched from outside; 'contacts' means sound and the rest, the sense-objects. Someone 'of unattached mind' toward those external contacts — whose inner faculty holds no attachment to them — such a person, free of any delight in sense-objects, 'finds' — obtains — whatever happiness there is in the self itself. That's the meaning. And such a person is one 'whose mind is joined in communion with Brahman' — union with Brahman, meaning absorption in it, is what 'communion with Brahman' means; joined to that, absorbed and given over to it, that's what it is to have a mind 'joined in communion with Brahman.' Such a person 'enjoys imperishable happiness' — obtains it completely. So one should draw the senses back from delight in external things, which never lasts, seeking instead the happiness within the self that never runs out. That's the meaning.
Close to the Sanskrit
And what is more, established in Brahman —
'In external contacts' — external, and those contacts, 'external contacts,' [i.e.] they are so called because they are contacted; 'contacts,' [i.e.] sound and the rest, the sense-objects; 'in those' external contacts, — 'of unattached mind,' [i.e.] one whose inner faculty is unattached — this one 'of unattached mind,' being devoid of delight in sense-objects, 'finds,' obtains, 'in the self whatever happiness' [there is], he finds that. This is the meaning. 'He,' 'whose mind is joined in communion with Brahman' — union with Brahman, [i.e.] absorption, is 'communion with Brahman'; joined by that communion with Brahman, [i.e.] absorbed, engaged in that, whose mind, [i.e.] inner faculty, [is thus] is 'whose mind is joined in communion with Brahman' — 'enjoys imperishable happiness,' obtains it fully. Therefore one should withdraw the senses from delight in external objects, which is momentary, being one who seeks imperishable happiness in the self. That is the meaning.