So what is a starter?
A culture of naturally occurring yeast and lactic acid bacteria, obtained by the fermentation of flour and water. by refreshing this this pot with flour and water we culture this community of wild yeast and lactic acid bacteria in it, and in managing an optimal levels of microbes we can use it to inoculate flour and water to make bread.
At the sourdough school we use the starter to make a leaven with. This helps to build higher levels of microbes for more open crumb sourdough.