Reading the Lankavatara Sutra is a great explanation, & analysis of what Buddha meant.

Enlightenment requires 'to have gone beyond' (Tathāgata); which is to attain reaching levels of spiritual mastery, where we become one with the Universal Mind.

Recognizing that though we are part of a Oneness with the Universal Mind, the very Source of reality is selfless, and was before infinity - Which Buddha called Nirvana.

I find that the terminology 0neness (Zero-ness), is a simpler way to understand Nirvana; as if we're One, the Source is Zero, and in that Empty space Source has infinite potential.

How do we know we have enlightenment: When the bowl floats up stream, because we witnessed it.