Verse 47 had spoken of the special favours shown to the Israelites by Allah. Now, with Verse 49 begins the account of these favours.
Someone had made a prediction to the Pharaoh (فرعون) that a child was going to be born among the Israelites who would destroy his kingship. So, he began slaughtering all the male infants as soon as they were born. But he would spare the females, as there was nothing to fear from them, and, moreover, they could, on growing up, serve as maid-servants. So, even this leniency was motivated by self-interest. What the verse refers to as "a great trial" is either the slaughter of the sons - which was a calamity, and it is the quality of patience that is tested in a calamity - or the deliverance from the people of the Pharaoh - which was a blessing, and it is the quality of thankfulness which is tested when one receives a blessing.
The next verse gives us the details about this deliverance.