Congo red is called a "pH indicator". What does this mean?

A pH indicator is able to tell what level of pH the substance is at. It can tell how acidic a substance is. If a substance has more hydrogen ions than hydroxide ions then it is acidic. If it has more hydroxide ions that hydrogen ions it is an akaline. the acidic scale is from 0 to 14. lower numbers means more acidic and higher numbers means it is an alkaline.

Explanation:

pH stands for potential Hydrogen.