The pH of lemon juice is 2, and the pH of orange juice is 4. Which of these is more acidic, and by how much? What does this mean?

The pH is a 1 to 14 scale that measures how acidic or basic is a solution, at lowest number the solution will be acid, in 7 the solution will be neutral, and in higher values will be basic. It is a logarithmic scale, so with each number you move, the acidity or basicity will be increased by a factor of 10. So, lemon juice is more acidic than orange juice by a factor of 100.