Starter is a "natural yeast" that is an active colony of bacteria (the good kind) and wild yeast. It is like a child and needs to be fed daily, if you plan on using it all the time. Otherwise, since it is "alive," you must keep it in the refrigerator when you are not using it. Take it out a couple days before you plan on baking with it to give it a couple feeds and activate it from its resting state.
This took a few trials for me (like...2 months worth), to really understand what this meant. The best way to "feed" your starter is to discard a bunch (if it hasn't been fed in a while). Yes. Throw some out. Or add it to a "discard" jar and use the discard in different things like muffins, pancakes, or waffles. Then, using a food scale, which is integral to making this work, measure how much starter is left. Based on that, use a ratio of 1:2:2 (starter:flour:water) and mix well. I use King Arthur Bread Flour, exclusively, but feel free to try out different flours. This will alter the flavor of your starter. Some people use wheat, rye, or a combination of all. Cover with a tea towel, mark the level with a rubber band, and let rest at room temperature until the starter has doubled in size. You'll notice lots of bubbles. This is an ACTIVE starter. When it has doubled, or reached it's "peak," this is when you want to use this starter for breads and pizza doughs. After it has reached its peak, it will deflate.