It is very simple. The lower pH, the more acidic.
This is because pH is calculated by:
-log₁₀[H₃O⁺]
[H₃O⁺] means the concentration of H₃O⁺ ions meassured in number of ions per liter devided by 6.022 • 10²³.
Logaritmic math is complicated, but in this application it means that when [H₃O⁺] is increased by ten times, the resulting pH will be 1 lower.
H₃O⁺ is usually spelled as just H⁺ since it is H₂O with an extra H⁺. Due to natural processes, some H₂O in neutral water will send hydrogen back and forth, which gives a small ammount of OH⁻ and an equally small ammount of H₃O⁺. When inserted in the formula, this small ammout gives 7 as the result, which is indeed pH for neutral water.
When comparing pH 8 to pH 4.5, I calculated that pH 4.5 contains more than 3000 times more H₃O⁺ than pH 8.
