If you're familiar with the verb to "indicate," then indicator should be no problem. An indicator points to, suggests, or shows something. The arrow on a street sign is an indicator telling you which way to go.
"It's not the heat, it's the humidity!" In other words, temperature is only one indicator of how it feels outside. Air-pressure, humidity, wind-chill, and cloud cover are other important indicators of weather conditions. You might hear a lot from newscasters about "economic indicators." These are figures, such as the rate of unemployment or inflation, that indicate where we're headed economically. Good or bad, we need reliable indicators if we're going to recover economically. Not all the indicators say the same thing: some say we're going to be OK sooner than others.