"So they took soot from a furnace and stood before Pharaoh. Moses tossed it into the air, and festering boils broke out on people and animals. The magicians could not stand before Moses because of the boils that were on them and on all the Egyptians. But the Lord hardened Pharaoh's heart and he would not listen to Moses and Aaron, just as the Lord had said to Moses." NIV translation
The boils were a burning abscess that affected the entire body and affected the knees and legs to where the magicians could not even stand. Even though the plagues had started to affect the people directly, Pharaoh was still too proud to accept the will of God.
"Then the Lord said to Moses, 'Get up early in the morning, confront Pharaoh and say to Him, "This is what the Lord, the God of the Hebrews, says: Let my people go, so that they may worship me, or this time I will send the full force of my plagues against you and against your officials and your people, so you may know that there is no one like me in all the earth."'" NIV translation
Pharaoh is given another warning but this one is much stronger. With all that they had seen, it is hard to imagine what the "full force" would have meant to them. God says that the purpose of these plagues is so that Pharaoh and all the others could see that there was no one like Him. They were used to having all sorts of gods but were being put on notice that there was none other than God.
"'"For by now I could have stretched out my hand and struck you and your people with a plague that would have wiped you off the earth. But I have raised you up for this very purpose, that I might show you my power and that my name might be proclaimed in all the earth."'" NIV translation
Pharaoh is reminded of the fact that God could have destroyed him and his people with a single act. Instead, He has used them as a demonstration of His power to glorify His name.
The same thing is true for us before we accept Jesus Christ. God would have been just to wipe us off the earth but He demonstrates His mercy to Glorify His name.