(Musa) said;"O my Lord! Open for me my chest, and ease my task for me."
Musa requested his Lord to expand his chest for his mission. For verily, He was commanding him with a great task and a weighty affair. He was sending him to the mightiest king on the face of the earth at that time. He was the most arrogant and severe of all people in his disbelief, and He had the largest army and the most powerful kingdom. He was the most tyrannical and the most obstinate of rulers. His case was such that he claimed not to know Allah at all, and that he knew of no god for his subjects other than himself.
Along with this, Musa lived in his home for a period of time as a child. He stayed in Fir`awn's own room and slept on his bed. Then, after this, he killed one of their people and feared that they would retaliate by killing him in return. Thus, he fled from them and remained an outlaw during this entire time. Then, after all of this, His Lord sent him to them as a warner calling them to worship Allah alone, without associating partners with Him.
This is why he said,
قَالَ رَبِّ اشْرَحْ لِي صَدْرِي
وَيَسِّرْ لِي أَمْرِي
O my Lord! Open for me my chest, and ease my task for me.
This means, "I cannot perform this task if You do not help me, aid me and support me."
وَاحْلُلْ عُقْدَةً مِّن لِّسَانِي