what does "I didn't know" mean?
it means "I did not know but now I know" or "I did not know and now I do not know" or "I did not know (we do not know whether he knows or not now)"

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Answer

I did not know.  "Didn't" is a contraction (shortening of) of the words "did not," which is the past tense form of the words "do not."  
Example: I arrived at the airport at 8:00 because I did not know (didn't know) the departure time was changed to 10:00.    
Hope that helps.^^

Answer

It just means that the person did not know what was needed to be done. (Or whatever the boy said he did not know to.)