What exactly is grace, and how can it be so profoundly sufficient?

In 2 Corinthians, Paul mentions a "thorn in his flesh," and he shares how Jesus told him, "my grace is sufficient for you." When I read that, it was like a light bulb suddenly illuminated my mind! If grace was powerful enough to uphold Paul in his darkest struggles, then surely it can lift me in my own challenges as well. But this leads to an exhilarating question: What exactly is grace, and how can it be so profoundly sufficient?