upaniṣad-śabde yei mukhya artha haya
sei artha mukhya, — vyāsa-sūtre saba kaya
upaniṣad - of the Vedas; śabde - by the words; yei - whatever; mukhya - direct; artha - meaning; haya - is; sei - that; artha - meaning; mukhya - chief; vyāsa-sūtre - in the Vedānta-sūtra; saba - all; kaya - describes.
Śrīla Bhaktisiddhānta Sarasvatī has explained the word upaniṣad in his Anubhāṣya. Please refer to Ādi-līlā, second chapter, fifth verse, and Ādi-līlā, seventh chapter, verses 106 and 108, for his explanation.