pH is the concentration of Hydrogen ions (shown as [H+]) in a solution. The scale for pH is 0-14, with 0 being very high acidity and 14 being low acidity (basic). The scale is a logarithmic scale, so this means that each number is 10 times stronger than the number that precedes it.
The pH of the tomato juice is 4, which is more acidic than the black coffee with a pH of 5. Since there is a difference of one number between the two (scale of 1), then this tells us that the concentration of Hydrogen ions in the tomato juice is ten times GREATER than the [H+] of the black coffee.
