Acids and alkalis are not all equally acidic or alkaline, so we need a way to compare them. The pH scale does this. It is a number that measures how acidic or alkaline a solution is, relative to others. The scale usually runs from 0 to 14.
A solution with a pH below 7 is acidic, and the lower the number, the more acidic the solution is. A solution with a pH above 7 is alkaline, and the higher the number, the more alkaline it is. A solution with a pH of exactly 7 is neutral. Pure water is neutral.
This lets us compare solutions. A solution of pH 3 is more acidic than one of pH 6, and a solution of pH 13 is more alkaline than one of pH 9. Only the number on the pH scale is needed to say which is which.