In the simplest terms, the value of pH simply gives us the absolute value of the exponent of the hydrogen ion concentration. On the pH scale, a 7 is considered to be neutral. Strong acids have pH much lower than 7. Some examples are given in the graph. In animals, for example, the maintainence of blood pH is crucial for life.
Lysosomes contain enzymes that function optimally in an acidic environment.
Such an acidic environment, however, would be detrimental to biological processes in the cytoplasm. Cells can maintain pH chemically by using buffers. Buffers are molecules that easily interconvert between acidic and basic forms, donating or accepting protons as conditions change. Notice the reverse happens as well. Buffer solutions are typically mixtures of a weak acid and its conjugate base.