It’s a reference to the original phrasing of Fermat’s note on said theorem: “It is impossible to separate a cube into two cubes, or a fourth power into two fourth powers, or in general, any power higher than the second, into two like powers. I have discovered a truly marvelous proof of this, which this margin is too narrow to contain.” (It is questionable whether this is true or not.)
It’s a reference to the original phrasing of Fermat’s note on said theorem: “It is impossible to separate a cube into two cubes, or a fourth power into two fourth powers, or in general, any power higher than the second, into two like powers. I have discovered a truly marvelous proof of this, which this margin is too narrow to contain.” (It is questionable whether this is true or not.)