This is the 6672nd most frequent French word.
just
"Just" in French means "fair," "right," "exact," or "just."
Here 'juste' means 'just' (recently or narrowly) emphasizing the recent occurrence of the event.
Je viens juste d'arriver.
I have just arrived.
In this context, 'juste' means 'just' (only/simply) highlighting the simplicity of expressing incredulity.
C'est juste incroyable.
It's just incredible.
In this sentence, 'juste' conveys the meaning of 'precisely/accurately,' indicating her exact positioning.