“Oh God, I have an ill-divining soul!”
This quote alone is referring to Juliet saying that she has a mind (soul), that predicts bad things for the future(ill-divining).
Ill-divining is a word used by Shakespear, spoken by Juliet, and broken down translates to; Ill is associated with sickness, darkness and death, whilst divining is having insight into the future and in their case their fate. Therefore put together it is saying that; the future she sees is dark.
In relation to the text, the quote is telling us that Juliet is sensing a dark fate for Romeo/herself, and this is confirmed because their fate is to die. It is just another hint of the future of the play.