Translation:

2. A Book revealed unto thee so let thy heart be oppressed no more by any difficulty on that account that with it thou mightest warn (the erring) and teach the believers.

Notes (Tafseer)

990. Heart: in the original, breast. I have used the word most appropriate to the English idiom. The meaning is that Al-Mustafa is consoled for all the difficulties which he encountered in his mission, with the fact that he had clear guidance in the Book for his preaching.