Heather, I think, Melissa means, "jong" is "attributive" form.

Like Afrikaanse (compared to Afrikaans), gouie (compared to goed) and so on.

PS. I feel a bit stupid answering, as you are from South Africa and I have mostly forgotten what I once learned of Afrikaans. I'm sure Melissa will answer herself. I only participate becuase there are not many people wiritng in Afrikaans section... so i think, the more, the better. I'd not forget it IF I had at elast soemone to speak ti with all these years.

PPS. when an adjective is describing some noun, it may change form: In "this girl is tall" and "she is a tall girl" a word 'tall' would look different. The latter use in linguistics is called 'attributive' use. 
Normally it happens with long (more than one syllable) adjectives. And in most cases it is just addition of -e.
But it also happens with some shorter words and sometimes it is not -e.

Sien jy daai jong man? Do you see that young man?

Jy is so jonk as wat jy voel! You are as young as you feel!

Dit is 'n baie lang gedig. It's a very long poem.

Hoe lank is jy? How tall are you?

Ek sou nie dit gedoen het as ek jy was nie. I wouldn't have done that if I were you.