From this day forward you will repeat what was done on day 2. Toss or save some of the starter and add 1 cup flour and 1/2 cup water.
What is happening is the natural yeast in your home are landing on the flour water mixture and are eating the flour, multiplying and releasing CO2 and bacteria.
A good starter generally takes 7-10 days before it is active enough to use for a sourdough loaf.
Troubleshooting. If you just can not seem to get anything going try adding some rye flour to your starter: 1/2 cup rye, 1/2 cup white, 1/2 water.
Rye flour has extra natural yeast that can jump start your starter. One other thing, Never and I mean NEVER add package/commercial yeast to your starter. They are like yeast on steroids and will take over your starter. The species of yeast in natural yeast is different than commercial.