In plain words
It means that those who can tell it have not experienced it. Uddhava came and set about teaching the Gopīs the science of yoga. Then the Gopīs: "O Uddhava! The sound of the music of Kṛṣṇa's flute fills our ears. It is not possible for us to take in any other words. Kṛṣṇa's enchanting form shines brightly in our eyes like lamps. Our eyes will not agree to see any other form. No other matter can enter our hearts. Go back the way you came," they said. That is, no other matter can enter at all into a heart filled with sacred divinity. For those who have fixed their gaze on one form, nothing else is seen at all. Waking, dream, and deep sleep: for devotees who have heard the divine voice of God even in these, no other sound reaches the ears at all. The heart of the Gopīs was always full of Kṛṣṇa.
In his eyes, always the look of compassion; his words always overflow with love; in his smiling face, the feeling of nectar; in the heart, always he brings joy.
This is the nature of one truly devoted. Today we keep saying, "Bhakti, Bhakti, Bhakti." We cannot show the derivation of the word Bhakti, the aim of Bhakti, its marks, or a form of Bhakti. Compared with such supreme Bhakti, whose form is complete, you may think for yourselves how small the Bhakti we practise today is. So Bhakti means this: keeping an aim, we must not let go our hold until we reach the goal that aim sets. We must not give up the vow. That is what we must take as a vow for life.