Indicators are vital tools in the laboratory. They give a cheap and easy way to check if a liquid is safe. You do not need expensive digital meters to get a quick result.
Most acid-base indicators exist as weak acids. We often write them as HIn. The protonated form and the deprotonated form have different colors.
This happens because they have different shapes and electron layouts. The ratio of these two forms changes as the pH changes. Each indicator has a specific pH transition range.
This range is usually about one pH unit above and below its pKa value. You must choose an indicator with a transition range that matches your experiment. The color change must happen right at the equivalence point of your titration.
The equivalence point is when the exact right amount of reactant is added. A common student mistake is thinking indicators change color instantly at one specific pH. The change is actually gradual.
The human eye only sees the full color switch when one form completely takes over. There are other types of indicators too. Redox indicators change color based on electrical potential. Adsorption indicators change color when a solid forms.