God's grace is sufficient for us. Grace in general terms is "favor." It is what God favors us with, what He gives to us. It can include spiritual gifts or physical things that He provides. By His grace, we have food to eat every day, we have clothes to wear, and we have a roof over our heads or cars to drive.
Paul considered his "thorn in the flesh" to be part of God's grace, a hard thing to say. How could he say that an affliction that God allowed could be part of His grace toward him? Because with an infirmity, whatever it happened to be, God balanced out for Paul the revelations that he had received, so that he would not become big-headed, sin presumptuously, and lose his salvation.
It was good for Paul to be afflicted, because if he were not afflicted, he just might have done something that he would have regretted, like presumptuously taking upon himself too much, more than had been given. So Paul says, "I'm content being afflicted, because I know that God's grace is sufficient for me. This affliction is good for me, helping me to make it into God's kingdom."
We have a hard time thinking this way. We consider this sort of affliction to be evil, but Paul turns that on its head, saying, "No, it is good, because with this affliction, I am weak, and because I am weak, then I don't get the big head. Then Christ can work in and through me, and the work gets done." So he was content.