So, the translation is a bit silly, and this is one of the most over- and mis-interpreted passages in the Pali canon. What it means is simply that the mind developed in meditation becomes radiant when the 5 hindrances are abandoned through the practice of jhana. This is obvious from full suttas that use the same terminology, eg. AN 3.101, AN 3.102, AN 5.23, SN 46.33.
The passage has had violence done to it by being severed from its natural context. This has happened as the passage has been incorporated in the Anguttara Ones. It is in fact fragmented across two occurences, at AN 1.49-50, and slightly more extended, at AN 1.51-52. The latter version gives a little more context. Both these sutta fragments occur in a long section of text fragments that deal with meditation, i.e. the development of the mind, especially jhana.
Now, in this collection we rarely have a proper “sutta”, if by sutta we mean a text that meaningfully represents a teaching given by the Buddha in a particular session. Rather, what we have are fragments, excerpted from other suttas, and sliced-and-diced to fit the format of being about “one” thing. In most cases this process is innocuous enough. But in this particular case it has imbued the isolated text fragment with a portentous flavor that has proven all too easy for those eager to latch Upanishadic style teachings on to.
Pabhassara means “radiant” (not “pure”!), and as with so much similar terminology in the suttas refers to the brightness of the mind freed from the hindrances.
Āgantuka is a unusual term in this context. It means literally “incoming” and usually is applied to a “visitor”, typically a monk or nun who is a new arrival in a monastery. The point here is that it refers to the hindrances as defilements that “pass by” the mind, as opposed say the anusaya which are underlying.
As for translation:
“This mind, mendicants, is radiant. But it’s corrupted by passing corruptions. An uneducated ordinary person does not truly understand this. So I say that an uneducated ordinary person has no development of the mind.”
“This mind, mendicants, is radiant. And it is freed from passing corruptions. An educated noble disciple truly understands this. So I say that an educated noble disciple has development of the mind.”
