The pH number indicates the concentration of hydrogen ions (H+) in a solution, which determines its acidity or alkalinity.

Here's a breakdown:

* Low pH (0-6): Acidic solutions have a high concentration of H+ ions.

* pH 7: Neutral solutions have an equal concentration of H+ and hydroxide ions (OH-).

* High pH (8-14): Alkaline (or basic) solutions have a low concentration of H+ ions.

Therefore, a low pH indicates a higher acidity, while a high pH indicates a higher alkalinity.