Answer
pH is the negative logarithm of the hydronium ion concentration:
$pH = -log[H_3O^+]$
We choose to discuss the acidity by pH representation because, it is easier to represent, if compared to the concentration of hydronium ion, which is, most of the time, a very small number.
Work Step by Step
You can find this information in every textbook that talks about acids and bases, or in the internet.
Example of pH representation:
$[H_3O^+] = 10^{-7}$ is represented by: $pH = 7$, which is a very simple way to describe the same information.
