What's an index

Hi there guys, this is my first post. I've been using the cAlgo system for a few days now and trying to code an indicator. I can't seem to find a decent resource where I can find an answer to a simple question. I'm struggling with the idea of index's. In the calculate method an index is passed in which is used with MarketSeries, then there is the GetIndexByDate() method and I'm just so confused on how it all fits together.

If someone could please explain was a series and index is and the difference between the two. Also, when the Calculate() method is called, which value is typically passed in and does it depend on which timeframe you're using and how can you differentiate between the value. You help is greatly appreciated.