Indicators are weak acids or bases that change color based on the solution's pH. The color change happens because the indicator exists in an equilibrium between two forms, each having a different color. The goal is to choose an indicator whose transition range closely matches the equivalence point pH of the titration. For example, phenolphthalein is ideal for weak acid-strong base titrations (equivalence point pH > 7), while methyl orange is better for strong acid-weak base titrations (equivalence point pH < 7).