What is pH?  The everyday answer:
pH is a short form for the power (p) of Hydrogen (H), which is a way of expressing the measurement of the hydrogen ion concentration. The pH scale is the way to express how acidic or basic a solution is. The pH scale is 0-14, where a pH of 0 to 6 (or 6.9) is a acidic, 7 (or +/- 0.9 around 7) is neutral, and a pH above 8 (or 7.1) to 14 is basic or alkali. Because ions come in massive numbers the measurements are made using logarithms, so pH 7 is neutral, pH 6 has 10 times more Hydrogen (H), pH 5 has 100 times more Hydrogen (H) than 7, . . . until pH 0 has 10,000,000 times more Hydrogen (H) than 7.

What is pH? The Chemists expanded answer:
A pH sensor will measure the activity of the hydrogen ion. Since the activity of the hydrogen ion is nearly identical to the concentration of the hydrogen ion, the practical definition is p[H] = -log[H+]. The concentration of hydrogen ion typically lies between 10E-14 and 10E0 mol/L. It is therefore convenient to express hydrogen ion concentration using the pH scale where p becomes an operator which says: “take the base 10 logarithm and multiply by -1.”