What is an indicator

Question

im not understanding 

Answer 1

In general, an indicator is a device, either mechanical, electrical, or chemical, that reads back a detected or measured value based on the calibration in response to its environment.  A meter is a great example of an indicator.

Answer 2

A pH indicator? It's a chemical compound added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually. Basically, it detects the presence of either hydronium ions (H3O+) or hydrogen ions (H+) in the solution and changes color accordingly as it reacts. When we observe that color change, we have an idea of the relative pH range of the solution.