Moses and Aaron went to Pharaoh and requested that he let the Israelites go so that they could hold a feast unto God in the wilderness. Of course the intent was to leave Egypt for good, but this method was a tactful way to begin approaching Pharaoh. God had told Moses earlier that Pharaoh would refuse.
The Hebrews had been using supplied straw. Now they had to gather the straw themselves. The gathering would take time, yet the number of bricks could not be decreased. Pharaoh wanted to so burden the Hebrews that they would not even think of asking for liberty to sacrifice religiously.
What is the spiritual lesson? When we give our hearts to the Lord or even if we are just contemplating giving our hearts, the Adversary uses every possible means to distract or sidetrack us into another avenue that will occupy our time. Pharaoh is a picture of Satan here. If Satan sees someone seeking liberty through Christ from the burden of sin and death, he specially tries to make it difficult for that individual to pursue consecration. The people are already burdened, and he increases the burdens.